You cannot "port" a supplementary line, you will have to convert the supplementary line into a primary line then you will be able to port the selected number you wanted

You mentioned that the Celcom number is a primary line, then clear your Celcom bill, then go to any Maxis Center and create a porting request from Celcom to Maxis.

Just create today?
If i'm not mistaken, number need to be 30 days with 1 telco before can port to another telco.

There is a one month cooling period for all operators if you mnp over. Which means, you have to wait one month before you can port out to another telco.

The easiest is port celcom to hotlink prepaid 1st.
Once successfully become hotlink prepaid, same day u can just go to maxis and request them to change the prepaid number to be your postpaid supp line.

Maxis is super duper suck like shit!
B4 ayam change to maxis konon2 want to downgrade monthly payments bill bcoz ayam have already registered on my name but the number used by my wife. Ayam only can tahan for 1 month. Very2 shit this maxis line. Ayam don’t know why this maxis still exist. Now ayam change to celcom back.