Even if you turned it back at this point, it still wouldn’t work.
This is pretty infuriating though; Google works just fine with any device that doesn’t run Android so why would they care that you’re running a custom ROM?
My guess is something less evil and more mundane: something about your number changed in their system and now they can’t send codes to it, which is why it’s grayed out. Maybe it was previously classified as a mobile number but now is classified as a landline.
Your only option, if you don’t have any backup codes, is to use that “Get Help” option they have that takes a few days and then either start carrying around backup codes, a Yubikey, or De-Google.
Hey, maybe all 3!
If you can afford it, I personally highly recommend just buying two USB-C hardware Yubikeys and storing it on there. Built in back up and if you put it on your keyring, it’s always nearby. You can still use your phone to access the code as well.
Not exactly super helpful for solving this problem but since you’re already going to have to reset or get new accounts, it’s a good time to switch over if you’re interested.