The factory reset idea is mostly to clear out any unauthorized customization that may have been made. If you can confirm that hasn’t happened then it wouldn’t be necessary. I have a router that’s not supported by my ISP so I feel your pain. Fortunately I only had to figure out how to tag a particular vlan on the WAN to get it working and someone else had posted a guide that got me most of the way there.
Can you get into your router’s admin interface? At the very least assuming you don’t have much networking experience I’d do these things in this order:
1 - Check for firmware updates and apply them
2 - Factory reset
3 - Change password
4 - Recheck for updates in case the reset wiped them out
There’s a million other things you can do to get more info on what’s going on and put in security layers to do this and that. But if you just want the maximum results for the minimum effort this is the best place to start.
Do you have any IoT devices chewing up a lot more bandwidth than they should be?