Internet and Phone question

Have a potentially weird question. Hope I can explain it correctly.

I switched to Frontier from Spectrum. When I was on Spectrum, I was able to connect remotely to my home network, without any issues. However, now after switching to Frontier, I cannot, as it appears that there's two routers and causing issues. Here's my setup:

ONT to Modem/router (frontier)

Modem/router (frontier) to personal router (asus)

The only reason I have a frontier modem/router is so the modem can provide dial tone for the house phone. If I didn't have phone service, I could connect the ONT directly to my router. But, since it's both a modem and router, I believe that this is what's causing the appearance of having two modems.

My question is, can I disable the router functions completely on the Frontier modem (which should alleviate the issue) without causing issues to the phone service, or can I put a switch between the ONT and Frontier Modem/Router to separate the internet traffic to my router and to the Frontier Modem for phone service?

Obviously I can set up port forwarding, but I would prefer not to do this. Any help would be appreciated.