3 of 5
provided a solution to my problem.

How do carriers block tethering?

I have two sim cards with data bundles from different carriers. One is working fine with (WiFi) tethering, but with the second SIM card I cannot use tethering.

Searching online I saw that tethering can be blocked by blocking browsers' user-agent headers. Alternatively using torrenting software headers and other PC-related application headers. In my case, I cannot even ping 8.8.8.8 from my laptop using the second SIM card, while I can from my android using a terminal emulator.

Tethering problem with the second SIM card existed from the beginning. Perhaps I just loaded one web page with it (or it seemed like loading, but in reality browser retrieved it from its cache).

So in my case, how does the carrier block the tethering? Of course, the purpose of this question is to bypass this block. There is also the possibility that there is no block at all, but something else is causing the problem, but I am baffled because this should happen with both SIM cards, right?

EDIT (solution): After paying $20 to my working SIM card to google around, I solved it myself... I discovered that USB tethering was possible with the second SIM card. I found this strange, and checked (Android 6.0) the 'Mobile data > Mobile networks > Access point names > ''carrier name'' Internet (or sth similar)' for both SIM cards.

This is a long list of settings (Name, APN, Proxy, Port, etc...). The difference was in the 'APN type'. In the working SIM it was filled with 'default,dun,mms,supl'. In the "faulty" SIM with 'mms' only! So I wrote 'default,dun,mms,supl' and now tethering works fine with a WiFi hotspot.