Modem/Router vs stand alone modem and router

The obvious benefit is there is one fewer box to find space for, plug into the plug strip, etc.

The negative is when (if?) the ISP upgrades service to DOCSIS3.1, you are stuck with a DOCSIS3.0 modem.  Or if the ISP offers fiber, the modem is no longer useful. (no coax = no need for modem).

This has nothing to do with WiFi coverage.  Coverage will be the same with an RBR40 as with a CBR40.  Something else affecting the outcome.

Diagnosis typically starts with asking how far apart the router and satellites are and how far WiFi devices are from them.

When satellites are too close to the router, they have great "backhaul" speed, but the WiFi channels tend to interfere and the total coverage area is small because of so much overlap.  When satellites are too far from the router, total coverage area is larger, but the backhaul speed is reduced.  As a general practice, most people recommend starting with the satellites about 30ft. from the router and adjusting from there.  (Obviously, if 30 ends up in the middle a room, you have to go where the furniture and the electrical outlets are.)  Anything massive or metal that is directly between the Orbis will affect performance.  Concrete walls, fireplaces, brick chimmneys, large appliances (like refrigerators), metal HVAC duckwork, metallic film on windows.  Lots of things affect signal strength.

Perhaps you could describe more about the physical layout.