Slower wifi Performance with purchased modem

Replaced my Technicolor(Thompson) cgm4140com that i was renting from Comcast.

Bought the ARRIS SBG7400AC2.  Both are 2.4 and 5.0 radios.

Configured it the same way.  If i am standing next to the modem, i get similar speeds of 230Mbps

But if i walk down the hall, it drops dramatically with my new modem.  about 30 feet distance and one wall penetration.

The Technicolor was able to maintain the 230Mbps when i moved to same location in hall.

What is the exact configuration for wifi antennas device?  Is it a 2x2 or some other varient of mimo?  Does the Technicolor remain on 5.0 even when you move away?

The ARRIS modem will switch to 2.4 when moving away, thinking it is the better signal, but 2.4 is a slower connection.  Are there specific advanced wifi configurations set that are giving this higher throughput?  Help!

Is Comcast providing higher server for modems that they rent?????