the Frontiers router does not NEED to be set in bridge mode to work with the Cisco Meraki
routers are configured in bridge mode to resolve problems with port-mapping/NAT (incoming or outgoing)
if you do not configure them in bridge mode then both the ISP's router performs NAT AND your local router performs NAT
this double-NAT is more difficult to configure correctly and sometimes give issues that are not clear
the Cisco Meraki needs internet access to contact the Meraki cloud management environment
I expect this will be no problem with your ISP router not in bridge mode
this will also be no problem for normal internet access for devices behind the Cisco Meraki router
