@Hypnoticfart
The equipment that AT&T provides to its customers is described here:
https://www.att.com/support/article/u-verse-high-speed-internet/KM1011652/
I don't think you can bypass AT&T Wi-Fi gateway.
There's two options to use your AX50 in conjunction with AT&T Wi-Fi Gateway:
1) Put the AT&T gateway in IP Passtrough mode and use AX50 in the default wireless router mode (WAN port set as Dynamic IP):
https://forums.att.com/conversations/att-internet-equipment/bridgemode-vs-ip-passthrough-setup-information/5defbfffbad5f2f606ad5ed2
If you contact AT&T support and explain your goal, I am pretty sure they will help you with the details.
In this case it's better to disable AT&T gateway Wi-Fi to minimize the wireless interference for AX50.
2) Use the AT&T gateway in normal mode and set AX50 in AP mode (Case 1) to extend the wireless coverage in your home.