Apple ID linked phone number doesn't receive verification codes

I previously owned an iPhone and my Apple ID was successfully linked with that respective phone number. Then I shifted to Android. Recently, I got an iPad and I logged into my Apple ID successfully. After a while, I was unable to use Facetime and iMessage. It asked me to Sign in again, which I did but as I had 2 factor authentication enabled, it said to enter the verification code sent to my phone number which I failed to receive multiple times. I reset my iPad and tried to log in. The problem persisted. I finally requested Account Recovery.

After waiting for a period of 24-hours, followed by 4 more days, my request for a new number to be updated into my account was accepted. I was able to log in and add another number to my Apple ID. That's great. But my main personal number which was previously linked to the Apple ID can't be added to my account. It repeatedly says "Error Description not available" when I try to add it to my account and it doesn't seem to receive any verification code or calls with a code. Please can anyone guide me regards to this issue. In the future, I want to shift to iPhone but my main number isn't linking with my Apple ID.

PS I was successfully able to enter two numbers in total to my Apple ID but my former number is not adding to my account.