Yes you will be able to access the att modem web page. Don’t forget to turn off the att firewall, all traffic shaping and anything else crazy that box likes to do. If you have the BGW210 box then you would be able to extract the certs and get rid of the modem in the connection path all together. I do this with mine and my att modem sits in the closet. The Pfsense Negotiates everything and gets it’s public up without issue.