I'm a little surprised your phone company chose to use passwords, but that's government for you.
Since your login info wasn't wiped, it sounds to me like you did not actually reset your modem. As I said, I have seen modems with a button right where I would expect a reset button to be. When I pushed it, the lights changed. But, it turned out not to be a reset button. It was an enable/disable button.
So, again, was the button you pushed actually labeled "Reset?" If it's not labeled at all, you can look up your modem model and look at a manual that will tell you the purpose of that button.
You can also use the manual to look at the Ports page in your admin screen and verify that it's all set to the default. In most routers, that will be blank. Some may list the standard ports, like 80 for http, etc. As long as everything is default, and once you reset your PC, you should be safe.
For the reset itself, it doesn't matter if your PC is connected to a network. Might as well just leave it connected, because after doing the reset, your PC is going to have to run through all the updates to get it back to current. So, it will need to be connected for that. I haven't done that with a Win 10 machine, lately, but it usually takes a couple of hours and multiple reboots. Keep going into "Check for Updates" even after you think you're finished. It'll download 108 updates and install those. It reboots. It gets back to the home screen. Then you go into "Check for Updates" again and there are a new set of 57 to download. So, keep going until you "Check for Updates" and it says you're up-to-date. Unfortunately, you do have to babysit it, because sometimes the major updates make you click an "Accept" box or two to continue. I usually surf on another machine or watch TV while I'm waiting for Windows to finish.
After all the updates, your first installation should be an anti-virus program. Then, I go to Ninite and install the common programs I like in one fell swoop.