The advice above matches mine.
I have a home-built pfSense unit and when parts die (not if), then I just replace them with spares I have already waiting… as that box is now critical for you.
I also have a Fritz in bridge mode with the pfSense doing PPPoE through it, so effectively, the firewall is the first real device on the WAN. Makes things much simpler as the WAN interface has status like packet drops, etc, much easier to diagnose issues.
I have a 7530. (Is yours a typo?)
Yes, those instructions look about right.
My pfSense box has the username & password, so the router really is just being used as a dumb modem (I used to use Draytek modems)…
… but…
The router’s diagnostics will show the DSL details, so you can check if your external connection is ok (ie OSI Layer1), but it will always think it’s offline.
So once you get your OPNSense setup and working, have a look around the Fritz diagnostics and get comfy with what you can / can’t see, because when there’s a failure you won’t know what is really failed.
Also… write down what you did and how to reverse it, as you (or others) might want to reset it to full router if your OPNSense is down.