Recently I've encountered a weird set of issues with wi-fi and NetworkManager (card is QCA6174, for those who might ask later, working under ath10k_pci driver)
I do not know how is it triggered (intervals seem random, with one exception), but on certain networks suddenly all TCP connections are timeout'ing, then the only temporary fix is to disconnect and reconnect to the same network, it works for minute or an hour, then it is back again to not connecting. UDP packets are coming as intended.
At first I suspected my DNS config, and yes, it was incorrect, but after correcting it there was no change of this situation.
Then I noticed that longer "sessions" of manually re-connecting using plasma-nm would cause it to stutter and freeze, sometimes along with rest of plasma UI.
I thought that the latest update might fix it all, that's why I was delaying this post, but now all affected networks are duplicated entries in plasma-nm along with device name in brackets after SSID.
And now about the exception - today I've found out that attempting to download anything with Steam client (e.g. steamvr update) triggers that timeout behavior near immediately, but this time plasma-nm also showing "!" near signal strength.
Edit: Almost forgot to tell this important information - other devices on the same wi-fi network, with same manual DNS settings, accessing same webpages at the time this occurs connect just fine.
