tmobile home internet speed issues

Im having issues after connecting a router to my Gateway.  I am averaging 200 download and 80 upload when everything is working.  if i connect a router to the gateway and turn on the gateway the secondary signal (5G) will not connect.   then my speeds are 80 down and 8 up when only connected to the primary signal (4G).  if i disconnect the router and reboot the gateway the secondary signal will come back and ill have the faster speed.   Now,  when the gateway is booted up and i have secondary signal.  I can then plug the router into the gateway and it will stay connected the secondary signal for part of the day..  but after a while the secondary signal will drop out again..  then it will come and go but it will stay disconnect more then connected….   then when i have no secondary signal i reboot the gateway without the router connected the secondary signal will come right back…. anyone with the speed problems have a router connect to their gateway?  I can recreate this issue every time.  i called t-mobile and had them watch my gateway as i went trough the steps but they dont have an answer..   I have the latest firm, and have good signal.