VMG3925-B10B 5GHz wireless unstable, again

Overnight the router decided to update itself and in the process killed the wireless. 2.4 GHz is solid, like it's always been, but it's slow, like it's always been. On default settings 5 GHz only appears in available networks briefly, but when my laptop (Win10) tries to connect it sits on "connecting" for several minutes and then the network disappears completely. I can't even get it to show up on my phone (Android).

The 5 GHz has worked flawlessly for months. Now it switches between completely dead and not really usable. I reset the router and set up the wireless again, but it has made no difference. There are a ton of other wireless networks in my building (big dense apartment building) so I always have the wifi protection on Auto because it seems to help the stability.

Locking the 5 GHz to 20/40 MHz makes it stay on, but it has a very noticeable performance issue (like go/stop operation) when copying files across the network.