Correct LAN IP to use

I would like to connect a Flexy205 to a PLC network for remote access. Flexy will use WiFi for internet access.

PLC network is has addresses of 192.168.20.xxx and 192.168.21.xxx
what is the proper way to configure the LAN address of the Flexy to see all devices on both address ranges?

You would have to assign the Flexy’s LAN in the 255.255.0.0 subnet and not have the WAN side in the same subnet.

So something to the effect of

Flexy LAN IP of 192.168.20.20 with a subnet of 255.255.0.0 and a completely separate WAN range would be fine.

That would be correct. You can also narrow it more to 192.168.20.20/22 (255.255.252.0) which would give you a subnet range of 192.168.20.1 - 192.168.23.254.