You won't find any of the mobile operators offering unlimited data mobile broadband products. They are not aiming to provide products that are a replacement for fixed line or fixed wireless broadband and their networks couldn't cope if users had no reason to moderate their data usage. This is unfortunate for people in your position where the only high speed service is from a mobile operator.
Unlimited data on phone contracts is subject to various terms. The networks usually attempt to block dongles and routers based on IMEI (the hardware serial number, in effect). Three also are believed to use deep packet inspection to identify phone SIMs being used to connect multiple computers, for example by traffic to Windows Update. There are also limits on how much data you can reasonably consume using a phone as a phone - even heavy users of video streaming will be unlikely to exceed a few tens of GB per month. Someone using 100GB per month might find their usage being carefully scrutinised.
You may get away with using a phone SIM in a router, though I have my doubts. If Three spot that your usage of a contract phone SIM has exceeded the monthly limit for tethering, they could send a hefty retrospective bill for excess charges.
I would look at mobile broadband products that are designed for router use. If the highest consumer packages are inadequate, investigate the business products.
It might be worth your while investigating alternatives, particularly if you are farming and therefore running a business as there are sometimes offers for business hampered by poor connectivity. Satellite might be an option, though is not without drawbacks. It is also worth checking with your local council to see if there is any hope of improvements to fixed line broadband to your location via the BDUK scheme.