@alwayschasinghappyendings you can reset the admin password back to the default one by resetting the router. Mine have a button that you can press with a paper clip/pin on them for about 15 seconds.
Before you do that get a laptop and sit in the same room. Then google the brand of router and find the admin website.
If you do reset the admin password it means any device within your household with a static IP e.g. printer will need to be reset.
It isn't hard or complicated as there are websites and likely YouTube videos that can tell you what to do.
The biggest problem is that your DP should not go into a rage if you ask questions about what is going on as you are both adults. He should be happy to explain it to you what he's done particularly as you have children who need to be blocked from accessing certain websites. (I actually hope I never work with someone like your DP.)
I suggest due to his actions you evaluate seriously whether you should stay together as the only reason he has gone into a rage is that he's accessing sites and content he should not be doing so, and likely tracking you.
You need to set up two factor authentication on all your email addresses, and have a password manager. (Again there are websites and YouTube videos to tell you what to do.)
Also in future if you want to get rid of a mobile phone factory reset it. Then discuss with your children once they are 14+ how you should get rid of your data on it and they can make money from your device, then once you have had the discussion give it to them to make money from it.