When it happens, have you tried pulling the WAN Ethernet cable and reconnecting it to see if that works? If it does, then perhaps there’s a similar issue using the default 2.5Ghz WAN port with your ISP’s router like I had with the Brume 2’s 2.5Ghz WAN port.
The problem I had with the 2.5Ghz WAN port on the Brume 2 was that it wouldn’t get an IP after a reboot which is the opposite of your situation. So I ended up swapping the 2.5Ghz WAN port around with the 1Ghz LAN port which fixed it.