Unable to access home server from external network; 443 is being blocked despite port forwarding

Hi. 

I'm helping my parents set up a home cloud server on an Ubuntu 18.04 desktop with the NextCloud cloud server. I can access the server through a browser on the desktop running the server (server is named "aslan") using the public IP ( 99.50.125.237) and I can connect using the domain I've set up at family.jackalope.tech using my hosted DNS service. I can not access the server from outside the local area network. I can only inconsistently connect to the server through phones or computers already on the LAN. Sometimes it works, sometimes it doesn't. I can never connect to the server through devices external to the server.

Using a port checker service ( https://portchecker.co/ ) I can see that port 80 is open and that port 443 is not. Port 443 is necessary in order to connect via HTTPS. I have gone through the router and set up port forwarding rules for 443. But it seems I can set up a port forwarding rule for one device per port. So that means I can open up the 443 port to a specific phone connected to the network, or a specific laptop. I can't however set up 443 to open to any device that attempts to connect though. Is this a correct understanding of the issue?

My parents have ATT Uverse with router BGW210. I have called ATT multiple times and the techs there did not seem to be able to understand what I was asking them about. I have also searched this forum and the information I have found has been sporadic and often incomplete. Can I get a clear answer on what the policy of ATT uverse is for opening port 443? I have read elsewhere that ISPs sometimes block 443 to prevent people from running home servers etc, but I am not clear on how true that is. 

Is there a way around this? Either by doing some kind of rerouting or adjustment of my server settings?