The legacy of 3.x still haunts us, why restrict ones world building to the statblock. Just because they cannot portal back and forth in combat, is no reason to believe that they cannot do it by other means.
It might take an elaborate ritual.
They have to apply to the Bureau of InterPlanar Transport for a ticket. They need to supply a valid reason why it will benefit to the Hells, or an authorisation from a recognised agent along with an expenses voucher or reference to a valid account. Everything properly notarised and in triplicate.
The prime would be completely overrun with devils but due to budget cuts there are very few currently allowed.
That Baldurs Gate thing blew out the whole years travel budget for Avernus. As for that devil Raphael, well if he was not reduced to a lemure by Tav, he would have been reduced to one for that scam he was running in black market transport chits.