@travller71,

First, have you tried waiting to see if the devices will come online after a period of time, or does the network require a restart each time? If it is an IP conflict, the devices would reconnect properly after their DHCP lease expires.

The devices should maintain reconnect to the router in a local-only setup and then reconnect to the internet as the router renegotiates with the modem. The IP addresses that are assigned should not change between having an internet connection and not. 

It does sound like the issue is with your router not correctly identifying that the modem has assigned an IP address to your network.  You may try to change the internet connection type and confirm with your ISP that you have chosen the correct option.