I'm assuming you can't log in from your primary PC because your home IP changed when you reset the modem for the new router? YES
everything else that was said i didnt understand any of it sorry
That sucks,
I think basically it means that since your IP address possibly changed Yahoo is doing extra security, I used to run into that with Hotmail which is now Outlook , I just transferred everything from that account to a new personal email address which is only used for private personal thing all other things like gaming or websites\ forums I belong to all use different email address's.
As for the blacklisted the other email address may have a filter that is blocking yahoo from getting through which would be pretty strange since Yahoo is pretty big company .
Sometimes sites for some strange reason just don't want to work for confirmation they send an email, but it never comes , I am currently going through this with Twitch I had to make a new account and actually created a new Gmail account just for that .
Proton mail isnt back for personal accounts
The first time it happened was my first email account I think that what happened was it was possibly hacked or attempted hack, so Hotmail locked the account ,then I made a second account and a few years later same thing happened. It may be possible that something similar happened to yow.
I still have my second ever email account which was from 2001 and yahoo I haven't had any real issues with it but like i mention below I dont use such places like Yahoo or Gmail even outlook for personal; things anymore
on a side not e Istill run into something similar with Gmail accounts I have a few for different things it's ok when I am signed in but if I reboot or sign in from my gaming PC or laptop I have to go through the same thing, but I also have it set up to send me a SMS to my cell phone