I'm in an almost identical situation to @vaindil , who posted the following (below ~ ~ ~) in Oct 2019. The reason I'm reposting his text is a) the time/version change since he posted and b) what worked for him hasn't worked for me!
I don't know if this is relevant but if anyone can help me urgently, my two kids are reliant on wireless to access school on Monday.
I've updated his excellent and mostly identical summary with my details (His original post was titled WRT1900ACS: Wireless suddenly has no internet):
~ ~ ~
I'm on a Linksys WRT3200ACM, latest version of OpenWrt (OpenWrt 19.07.7 r11306-c4a6851c72 - I was on 19.07.02 when I detected the issue but as per Vaindil's suggestion, I upgraded to the latest firmware). I've been using my router for months now with zero issues, haven't even had to restart it, but this morning when I woke up I discovered that my wireless networks have no internet connection. I have two networks, 2.4 and 5GHz, neither can connect to anything (not even the router control panel). My wired connection is working just fine.
I have restarted it three times but that's had no effect
I haven't changed anything at all in several weeks, so I'm not sure what's going on. Restarting the router didn't help and I'm at a loss for what else to check in my config?