Me and my daughter both use separate Mikrotik R11e-LTE6 modems on at&t's cellular internet. Neither one of us can reach or ping support.ubisoft.com. We can access the Ubisoft Connect stuff and friends list just fine, but we cannot connect to the Tom Clancy's Wildlands multiplayer servers. I have tried turning off all firewalls on my Windows 10 computer and my ASUS RT- AC3100 router, and there has been no change. Is it possible that there is a setting in the actual modem that is preventing our computers from accessing that server? Our modems are also updated to the latest version. I would be glad to share any other info that may help.

Good Morning,

1. The Ubisoft Server "support.ubisoft.com" does't reply to ping.
So don't worry about that !

2. NAT and Ports
Most Online-Games today need to communicate directly to your Computer.
So you will have to Foward some Ports and edit your firewall.

a quick google search :
https://portforward.com/ghost-recon-wildlands/

Port forwarding is a relic of the 00s and not necessary for any modern applications. Only if you want to host a server do you need port forwarding.

Most likely it is blocked by their ISP, cellular connections tend not to work great for anything except simple TCP/IP.

Agreed, if you are getting a private IP then unless the ISP forwards the port to your private IP, its unlikely that port forwarding would work.
As stated by RIch, most modern programs work becuase they are based on you joining a server (traffic initiated by you) and thus all return traffic is permitted.