You tried adding your MOBIB card to the Skipr app but it got refused? The cause might be very straightforward.
All Belgian public transport services sell MOBIB cards that give you the possibility of charging STIB-MIVB tickets on it. However, if you have never charged STIB's services on the card, it will not be linked to the STIB-MIVB. That is why you will not be able to add it to the Skipr app.
You can get your card activated by going to one of the STIB-MIVB offices and requesting this at the reception.