The hotspot feature is very questionable and grey.

Malaysians telcos are now very good at twisting words.

Hotspot means the SIM must still be inside your phone which you are using with your unlimited plan. If you take it out and insert it into a modem your speed will either get throttled, blocked/banned from connecting or you'll experienced web surfing limitations such as many ports being blocked and inaccessible.

Take for Hotlink Unlimited which claims to be hotspot friendly but when you place the SIM inside a dedicated modem, you can still connect but when the WiFi client tries to use Whatsapp you will experience weird limitations such as unable to upload files and make calls.