MR6110 AT&T FirstNet with external router/wireless access point

I've had this device for almost 2 weeks and I'm trying to help it work a bit less and more efficient.

I got a cheaper wireless router from BestBuy: LinkSys EA7200

I'm able to disable the DHCP server on the MR6110, and turn-on IP Passthrough. I've attempted manually setting the DNS. I've tried to modify the LTE Subnet Mask to see if I could get a couple of IPs (one for the modem and one for the router hoping this may bridge the gap in their hand shake process) and none of these settings seem to make the external router happy. 

I connect it to the internet port of the Linksys and it is unable to automatically configure. I've attempted bridge mode, and statically setting an IP. 

I've called AT&T asking for any login information that may be needed for PPPoE or LPPT their customer service didn't attempt to elevate the question but referred me to Netgear, who-- after about 1 hour referred me back to AT&T. So I'm here trying to see if someone can see what I've missed if anything. Thank you in advance!