Wifi Pro Extender not connecting to my gateway

Ive set it up 15ft away from the gateway, in a window, the activation ends in error each time. Chat customer care repeats the same steps and we get disconnected each time.  Ive replaced the extender and the modem.  Equipment should not be the issue.  What steps can be taken to get past the error? I feel like at this point it may be an issue with the heirarchy on the equipment on my account.  I recently moved and removed cable boxes from my account. One was showing not returned for a while. Is this causing the issue?  Please help!