LAN woes RBR50 with new att gateway

PLease advise if this is not the right place to post

I have an Orbi rbr50 with 2 satellites.

The master Orbi previously received a direct input from the modem & was connected to my iMac server via one of the outputs from the rbr50. Other devices were connected to my home LAN by wifi or by ethernet (mac mini) via cable running through one of the satellites.

I upgraded to ATT fiber 1 gig and I am now using the ATT gateway instead of the previous netgear modem.

When they installed the fiber, they could not install it in the same location as my previous service was located, so they installed it in another part of my house ie, where my mac mini is located. 

I moved the master Orbi to be near the internet source, the gateway, by the mac mini. The iMac, my server, was now connected to one of the Orbi satellites by ethernet.

The problem:

I can get internet at every location, but my home network LAN seems to have disappeared, or at least the the mini doesn't see it.

I connected the out port of the gateway to the Orbi master, and one of the other out ports from the gateway to the mini, so, to be clear the mini is not attached to the Orbi, but is directly attached to the gateway itself...would that config cause the mini to not see the network, essentially being out of the Orbi loop? Or is something else at play?

Appreciate any help on this. Both ATT and Apple were of no help, though they valiantly tried.

Thanks