How to configure TimeCapsule running behind Motorola NVG589 modem/router??

I just switched isp from Time Warner to ATT. ATT installed their 'new' Motorola NVG589 modem/router in front of my 3rd gen Time Capsule, to handle bundled tv/phone/internet. Since installation my TC network drops out randomly several times daily for 5-10 min , then self-corrects. Local ATT techs declare the modem/router clean/all green, and of course they can't support beyond their box. ATT ConnecTech (subscription!) support seems to be floundering after just resetting modem and TC channels from Automatic to non-overlapping channels. TC is set to Bridge mode. I have done factory default resets on the TC, but no joy.

Has anyone dealt with the NVG589/TimeCapsule configurations to avoid internet drop-outs? No way I want to lose TC functionality ! Many thanks.