Just make sure the system time on your router is correct, or the Wireguard server will refuse you to connect again.
The router usually doesn't have a RTC, so the system will be incorrect if you unplug the power for the router. Usually openwrt connect to an NTP server after boot to sync the time, then the time will be correct. But if the wireguard proxy all the traffic, the router can't reach the NTP server before the wireguard interface is connected, but the wireguard need a correct time to connect, so it is a dead loop.
To solve this, one solution is only let the Wireguard proxy all ipv4 traffic, and find and set a NTP server with support ipv6 on the router (in luci System/Time Synchronization).
Or you could apply a delay boot for the wireguard interface, give the router some time to done the time synchronization, before trying to connect to WireGuard server.