Most can't now especially the cheaper tiered ones on prepaid plans.

Even if they claim it's hotspot-able, it means the SIM must still be in the phone and you can only share it but turning on the hotspot feature not by inserting the SIM into a modem.

The telcos don't want to spoil their high priced fixed mobile plans with cheap prepaid plans which defeats the purpose. So to be safe the only mobile plans I can think of now which are modem friendly are:

Unifi Air Plan
Maxis Postpaid Family Unlimited RM99
Celcom Home Wireless Broadband
DiGi Old Infinite 150 Plan

Or you can try those quota volume based prepaid plans.