Is there any way to check if 4G/LTE is broken in LineageOS (FP2)?

Question

Situation

• I have a Fairphone 2. I recently installed Lineage OS 17.1

• I have a UK phone provider.

• I am in Germany for the next few weeks.

• I am not able to connect to German networks with 4G (that worked in the past and with the same contract when I was there the last time, before I installed Lineage OS)

• Instead it falls back to 3G and only stays connected for 5 minutes or so, then kicks me out. So mobile data practically does not work.

• In the UK, mobile data did work. I am not sure if it also fell back to 3G or if it did continue to use 4G after I installed Lineage. I cannot simply go back to the UK and check.

• I bought a local German prepaid SIM. It does connect to 3G, but not to 4G. It also regularly loses connection on 3G, especially when I'm indoors, but it stays connected most of the time, though only to 3G

• I know very little about how to debug Android-like OSes. To me it feels like being told to cross a busy road blindfolded: Everything is needlessly complicated, you can't see what's going on, and anything you do will probably end in failure.

• I would be happy if I somehow had mobile internet, preferrably reliably and preferrably with 4G. But failing that, 3G is also fine until they phase it out next year, if it works reliably (which it doesn't).

• I have switched the preferred network to 4G in the settings.

• I tried switching to 3G; it doesn't change anything.

• I tried rebooting the phone (the only trick I know for Android-like OSes where I cannot see what's going on); that doesn't change anything either.

Question

Since 4G never worked with neither SIM card since I tried to understand what's going on (i.e. since I am in Germany and discovered that mobile data did not work any more), my only working hypothesis is that 4G is broken in my setup. How can I test if this is actually the case?

Alternative possibilities include:

• German providers don't like UK contracts any more and refuse to provide roaming services (because of Brexit and everything)
• German providers do not like Lineage OS (or the other way around)
• German providers do not like FP2 (or the other way around)
• I am not smart enough to work with the kind of software Google has cooked up (Android like OSes) to figure out how to make it work with German networks
• I somehow randomly always have terrible phone connection (although this does not affect the people around me)
• My FP2 is somehow broken (and magically broke exactly when I crossed into Germany)
• Any number of other increasingly unlikely possibilities...

Update (6 weeks later)

Additional Facts

• I observed that I do occasionally - very rarely - get 4G/LTE. So from the technical side it should work.
• I tried 2 different German prepaid cards (Vodafone, O2) over the past weeks, both contracts should have enabled 4G/LTE. Almost always, I only get EDGE/3G (or a light version of 3G? It feels like in the 1990s.).
• I asked the salespersons in the Vodafone/O27Telekom shops. When they did not try to sell me random stuff, they were trying to be helpful. However, none of them had an explanation or could solve the problem.
• I updated my Lineage occasionally. No change.
• I cannot simply flash stock Android again. I only have this one phone and I am relying on the parts of it that do actually work and cannot afford to break it.
• According to this source, German providers use LTE Bands 1 (2100MHz), 3 (1800MHz) for Cities, 7 (2600MHz) for cities, 20 (800MHz) especially for the countryside. Occasionally Bands 8 (900MHz) and 28 (700MHz) are also used, but this is limited to certain providers or regions. FP2 can only do Bands 3, 7, 20 according to acejavelin's comment.

This eliminates some of the possible explanations above. The ones that remain are:

• German providers do not like FP2
• German providers do not like Lineage
• My Phone is broken
• My installation is broken
• German providers downgrade non-privileged contracts (foreign and prepaid) to 3G/EDGE, rendering them essentially unusable.
• FP2 only supports LTE Bands 3 (1800 MHz), 7 (2600 MHz), 20 (800 MHz) (acejavelin's suggestion), German providers may be using another Band since recently?

Regarding the question this thread was about (is there a way to figure out if 4G/LTE is broken)

• I assume the lack of answers means: No, there is not. If anything does not work, the cause has to be inferred from creatively designing experiments to rule out possible explanations one by one. E.g. installing stock Android and checking if 4G/LTE works there (acejavelin's suggestion).

Answer 1

I'm not sure if your issue was the same as me but I found your post because I had a similar issue. My 4G/LTE not working on my new FairePhone3 running LineageOS. I fixed my issue, here is the solution in case someone has the same problem.

My phone would show the 4G/LTE as working but zero bytes transfered and no apps could access the internet. I double-checked the APN and everything seemed fine.

I activated dev options and USB debug. From a computer I ran the following command (the phone being linked by USB) :

adb logcat -b radio

In the logs I saw some of the following things :

[...]
PDN_IPV4_CALL_DISALLOWED
[...]
01-30 19:37:40.196  1970  1970 D RetryManager: [default] Reached maximum retry count 10.
01-30 19:37:40.196  1970  1970 D GsmCdmaPhone: [0] getDataConnectionState apnType=default ret=DISCONNECTED
[...]
01-30 19:37:40.198  1970  2203 D DC-C-1  : updateLinkBandwidths: LTE
01-30 19:37:40.199  1970  1970 D DCT-C-0 : isEmergency: result=false
01-30 19:37:40.199  1970  2203 D DC-C-1  : DcInactiveState: clearing settings
01-30 19:37:40.199  1970  2203 D DC-C-1  : clearSettings
01-30 19:37:40.205  1970  1970 D DC-C-1  : updateLinkBandwidths: LTE
01-30 19:37:40.205  1970  1970 D DCT-C-0 : onDataSetupCompleteError: Stop retrying APNs. delay=-1, requestType=NORMAL
[...]

So the logs confirmed I wasn't crazy, obviously there was an APN problem. The IPV4_CALL_DISALLOWED stood out to me. In my APN setting I found a APN protocol option that was set to IPv4, I changed that to IPv4/IPv6 and now my 4G/LTE works again !