Roger Mills laid this down on his screen :
Are there any gotchas when it comes to data SIMs? For example, PlusNet told
me in an online chat that their SIMs can be used in phones or tablets but not
routers - but I'm not sure that the agent knew what he was talking about.

Prior to my phone having 4Gb of data per month and my phone having a
personal hotspot facility, I used something similar with a Three data
sim to provide my laptop with access, whilst away in my tourer caravan.
I put a sim in it and it provided wifi access.

From what I read, Plusnet have no objections to a phone being used as
an hotspot and there is zero difference between an hotspot and your
mobile router.