For anyone having this issue in 2024, what I did was setup authenticator and turn on sms notifications (sms protect is nowhere to be found at this point) and made sure my phone number was correct. It still didn’t seem to work. So then I logged out of wow, made sure to back out to main screen, log out from there if required, then exit game. Then log out of battle.net. Then go to web account page and use the log out of all devices options. Then, logged back into battle.net, then went into Wow, and bam, I got my 4 extra bag slots!
I’m playing a timerunner account and had 4 timerunner satchels with 36 slots each… but man I really wanted that 4 extra slots in the main backpack and didn’t like that plus sign hanging around 
Glad it’s no more.