I bought a Three standard phone SIM with unlimited calls and SMS and 100GB data per month and have been using that in my ZTE286D router for about four months now.
I get decent speeds of up to 60Mbps down 40 Mbps up on 4G, about a mile or so from the nearest base station. I don't believe that a data only SIM would give me better speeds but I doubt that it would. Anyway, i am quite pleased with what I get compared to Plusnet ADSL which I had before.
Data SIMs turn out to be more expensive, e.g. 120GB of data only on Three at £20/month. A Three "phone" SIM with 100GB of data costs me £7 per month after cashback. There is a good website to compare SIM deals here
https://www.smartphonechecker.co.uk/sim-only
Having said that, there is a company that sells prepaid data only SIMs giving you a certain amount of data every month until the card expires e.g.
https://www.amazon.co.uk/Three-500GB-Data-SIM-Preloa... which works out even cheaper but you have to cough up a bundle of meoney up front.