@SunnyvaleJim 

If the T-Mobile 5G gateway can be setup to run as a backup service with fail-over detection then this is a simple solution but you'll be double NAT'ing things.  The Xfinity line (I'm assuming you are just taking the RJ45 Ethernet cable from the back of your cable modem into the Deco units currently) would go into the T-Mobile WAN port and you would then connect a Deco unit to a LAN port on the T-mobile device.  I've done this in the past with some TP-Link 4G/5G devices, and most vendros (TP-Link and others) support this sort of functionality.

I'd describe this as an in-line or series solution.

The alternative if you need dual simultaneous active connections would be the parallel topology you mention - add a box that can support 2 or more WAN connections and then plug the Deco's into a LAN connection on this box (plug for the TP-Link Omada range here if you are looking for something to consider, but its whatever you have access to or are comfortable with). 

  @meshing_it Thanks! Which Omada product do you recommend for the dual WAN ports? 

  @SunnyvaleJim 

Hi there, the Omada options you are asking for that I'd look at (all available in many different retailers & on-line) are;  

1) (entry level) the ER605, or

2) (a slightly beefier unit) the ER7206. 

With these units, the controller (the tool that you configure the Omada devices with) is either a software utility you download and put on a PC/server ( https://support.omadanetworks.com/us/product/omada-software-controller/ ) or you can get a dedicated hardware controller box (i.e. OC200).

Alternatively the ER7212PC is a combination of Gateway+ Controller Box and has some PoE ports - which is a very versatile unit, but costs a bit more.

Not sure if these have all the features you are looking for, but search these and others' similar products to compare specs to find what best matches your needs.

Hope these help you.