Hi, yes i’m using a FritzBox, too.
Lan1 (FritzBox) <-> WAN Port Shield
Lan1 (Shield) <-> HomeNetwork (for me, my switch)
Static IP on my FritzBox (For me 192.168.178.1)
Static IP on Shield (192.168.178.2); Gateway, DNS 192.168.1771
LAN Shield ((Router mode)192.168.177.1), my homenet is based on 192.168.177.x.
You could use the DHCP for assigning the IP’s in your homenetwork.