Let me just tell you this.

The ISPs can also whitelist only a handful of allowed devices which they permit and ban all rogue devices which is not in their whitelist from connecting to their network.

In the case of their broadband plans, they'd only whitelist popular known modems IMEI in the market which are widely used to only gain access to their network and other rogue devices blocked to prevent usage of their home broadband 5G plans to be used in regular phones.

About Yes, they're actually the pioneer in implementing this restriction in Malaysia. 2 years ago they already did this with their RM35 Kasi Up 4G 100GB Postpaid plan meant only for phones.

I was once their victim suddenly my 4G modem speed became so slow that you could get 500kbps when all the while obtaining 10-15Mbps.
Called them up and they straight told me my device wasn't in their green list.

It was actually an illegal move because nothing was said in their contract that the quota restricted plan was supposed to be for phones only.
The move was controversial and arguable that since it's quota based(100GB) why did you restrict your users in placing it into any device they wanted as long as they do not bypass the volume quota?