I have (at the office and at home) WiFi APs which serve 802.11n and 802.11ac over the same SSID.
My Galaxy S9+ never connects to the 5 GHz network on these SSIDs. I thought that this may be because the signal strength which is too low on the 5 GHz network and the 2 GHz looks more promising.
I created a 802.11ac-only SSID and connected my phone and tested for bandwidth which was ~90 Mbps, even in the most remote part of my home.
This probably means that the algorithm which selects the standard to use is less than optimal and I would like to help it by prioritizing the 802.11ac signal on a dual-standard SSID. Is this something which is configurable a way or another on Android (specifically Oreo on an S9+)?