I wanted to add I’ve been having the exact same issues. Using wireguard client for a year without any problems but suddenly the last couple days I cannot use it. I used the automatic method for both mullvad and azire and even tried manually inputting.
It will connect but always drops within 5 minutes. I wonder if wireguard was updated and the routers need an update as well.
This is not an isolated incident for the op. I changed nothing and it suddenly had issues.
Hopefully the issue can be found because it certainly does exist within the router.