the bug seems a validation issue in the GUI when SM is in NAT mode
WAN = NAT + DHCP so it has an IP assigned from the network
LAN = 192.168.0.1 + dhcp server, etc.. to service local network
==> complains about WAN and LAN can't be on the same network
if I change the WAN to NAT + Static IP => it shows 192.168.0.2 as default Static IP for WAN. This is the IP being incorrectly checked against the LAN IP.
If I put this IP as 192.168.99.2 then switch again to DHCP, I can put the 192.168.0.1 in LAN without issues.