From time to time (especially If I'm out & abut but technically working) I set my phone up as a wifi hotspot and connect my works laptop up to it then access the company systems via VPN.
Which is perfectly allowed under most plans. But no plan in the USA on ANY CARRIER (and probably there either) allows that to be done on an "unlimited" data plan AND pull from the "unlimited" high-speed data pool (it is usually a separate, capped, data pool). Which means you will hit caps.
Now, there are technical ways around it so it CAN be done, which requires unlocking the phone, rooting the phone with nefarious tools, installing special utilities. Doing so could then enable wifi hotspot access to your "unlimited" data pool- which will violate your terms of service (the contract you signed with your provider). And they can and do catch people. It is not necessarily criminal, just civil. So they could just warn you, they could terminate your service, they could ban you, they could damage your credit rating, and they could also sue you for lost revenue (I doubt that is ever done, though).
On MY T-Mobile plan, I have a older (grandfathered in) 6GB plan. So they don't care how I use the data because it is not "unlimited." Tethering from that pool works great, is very fast, and is built right into the phone- it is one, shared, high-speed data pool. But their standard "unlimited" phone plan includes only very slow, 3G tethering (wifi-hotspot). If you pay $10 more a month, you can then retain the "unlimited" high-speed data on the phone and also add 10GB of high-speed tethering (wifi-hotspot) and after 10GB, it drops to very slow 3G data speeds for tethering. And when I say slow, I mean SLOW.... like only a little faster than the fastest dialup modem speeds.
Verizon calls their main phone plan "unlimited" and then clearly indicate after 22GB it will drop to the "lowest priority speed." But they include 10GB of high-speed tethering data which after 10GB drops all tethering to 2G speed (which is like having no data at all). Want to use just tethering data with a dedicated hotspot? Sure thing! They have a plan for that device which is $110 for 20GB (for example). If you are trying to replace a home cable modem and have multiple users and stream HD video regularly and such... you are probably using WAY more than 20GB. But not to worry, they have a 50GB plan for $335 a month!
Sprint/AT&T are much the same.
You cannot get unlimited high-speed cellular data (LTE/4G) without breaking your terms of service (which is like service theft). At least not in the USA (which is what I am most familiar with) and I speculate it is the same for every carrier in the world.