I have unlimited cellular data on my phone but not if I use it as a hotspot.

They had this restriction in the UK where the networks would prevent hotspots from actually working. You had to buy a special additional package.

Restriction has now vanished and there are no such limits on usage. Not sure if the Regulator intervened but it was most certainly a cash grab.

These days they still manage to rip us off by annual contract increases of RPI+3.9%. That applies even during a 2 year contract.