Gateway Help

I have AT&T DSL Internet 18 (only AT&T service available in my area unfortunately) and the provided gateway.  I can't get good signal in my home as it was built in the 30s with plaster walls.  I'm a heavy internet user along with my other 3 family members.  I purchased the Orbi AX6000 RBK853 (1 Router, 2 Satellites) to expand my range, however, I can't figure out how to enable IP Passthrough.  Any help or alternative solutions for improved internet speeds/range would be greatly appreciated!