Mtn sim card stopped because router is not 5g

Mtn sim card stopped because router is not 5g I currently use an old 4g router I upgraded to a newer 5g router

I read on mybroadband.co.za that if you don't upgrade to a 5g router, then they block your sim card and tell you to get a new sim card. But even that sim card won't work in the older 4g routers, you have to have a 5g router

If mtn wants to upgrade me to 5g, then they should do it at their expense, not mines. MTN are a bunch of losers.