My ISP (frontier) refuses to do any support unless their router is connected, I have had issues in the past where I know 100% that it is the ONT that is the is

My ISP (frontier) refuses to do any support unless their router is connected, I have had issues in the past where I know 100% that it is the ONT that is the issue, yet I have to connect their router to it before they will do anything or send anyone to look at the issue The second they find out I am not using their hardware the call ends sadly even with the extra fee, I am still paying about 30% less for Symmetric fiber than I would for a similar asymmetric connection from Comcast which is the only other provider for me