To help make a comparision, I have a 4G Time Capsule as my "main" router in my network with a 4G AirPort Extreme configured in a roaming network. The connection between the modem and the TC is 1000 Mb/s, as well as, the connection between the TC & the AEBS. Both routers are also at firmware v7.6.3. The AirPort Utility is version 6.2.
What is the connection value in your case between your primary AEBS and the modem? Is it also 1000 Mb/s or is it 100 Mb/s?