On a WAN port, and the way zones are (WAN vs LAN), you want to have different L3 subnets otherwise you will create a routing problem. It needs to be able to ARP, and when it’s the same on both sides, it won’t ever leave the LAN, that’s why moving the port mode from WAN to Standard allows access.
You’ll either need to move the ONT address to separate address space or change the internal LANs so that there is no subnet collision causing routing issues.
EDIT: sorry about initial reply, wrong concept, similar premise.
EDIT2: The above quote may mention 192.168.11.0/24, but that is when the WAS-110 is at its default address. if the WAS-110 is changed to 192.168.50.x/24, then the internal network cannot be 192.168.50.0/24, etc.