I have precisely this deal, at the public price of £20pm. I believe you will have no problem whatsoever.
First, the publicity for the Black Friday deal made it quite clear it could be used for tethering.
Second, see this ISPreview article
of 4 October 2018. Specifically part of the quote from Ofcom:-
* already withdrawn restrictions on the use of handset SIMs in dongles and mifis.
Three has also confirmed that, from December 2018 (or earlier), it will:
* cease to restrict video quality to Standard Definition when its customers are roaming in the EU;
* remove any specific tethering or hotspot allowance for new or upgrading customers, to allow for unrestricted tethering (within the UK or EU);
* remove any requirements for customers to purchase a Data Passport to tether when roaming in the EU;
* allow customers on all Advanced Plans to tether without restriction; and
* allow customers on Essential Plans to tether on their current plan, or to migrate to the nearest equivalent Advanced Plan available at that time.
WIth both the above in mind I have tried it:
- in my phone and used directly in it;
- in my phone configured as a wifi hotspot;
- in my phone tethered by blue-tooth to my laptop;
- in my phone tethered via powered USB to my laptop and simultaneously providing a wifi hotspot for other devices;
- in my phone with the phone plugged into a powered USB slot on my ASUS RT-N66U router, with the phone acting as a modem (that also establishes the PPP session rather that the router doing so). The router then providing ethernet and wifi to my other devices.
My Three account for this (apparently phone rather than data) SIM refers to it as being inserted in my dongle
. So I assume it is in fact a data SIM but just not publicly declared as such.
I've used a couple of GB since receiving the Sim.
I hope that helps.
My broadband basic info/help site - www.robertos.me.uk.
Domains, site and mail hosting - Tsohost
Connection - Three 4G, tbb tests 35-45Mpbs down, 9-15 up.
If you never think of anything off the wall, you'll never think of anything original.