I can’t tell you what to do with it your situation. But maybe describing my experience can help you get what you want.
I have DSL with Hinet. They also provide a shitty router. The username / password they provide is a limited account. For the admin account they have a different login credentials. Only the technicians are supposed to know them. Previously those were leaked online. I was able to dump the config. From the file I extracted to DSL line dial-in credentials.
When using my own state of the art hardware I ran into issues. The DSL would drop out and had to sync up again quite often. I blame it to a non standard DSL protocol usage to keep out competition for Zyxel.
I had to go back to Zyxel router, but now I just use it to sync to dsl line. I PPPoE own connection to Hinet and can set up my home network how I want, with SIP phone line etc.