When you have two "NAT routers". Whether double NAT or not. wall--RJ11--router-modem(RM)---LAN-WAN--router(R)----LAN--comps (A Router-Modem(RM) so no WAN port, And a router R which has a WAN port, and ...
Regarding bridge mode e.g. one has a router-modem(as in 'home router' so it does NAT too and also a modem inside), and a router(i.e. a 'home/NAT router' with no modem), and one wants to connect them.