Like a lot of others, I use a mifi with a Three data sim. The only place it struggled was Lincoln showground. Amazon's the best place o get the sim, currently about £35 to £40 for 12gb which is valid for 12th months. It's also useable abroad.
We use a Huawei E5330 which is unlocked. I check the mobile network coverage for the area I am visiting and buy a 6gb data sim which is normally enough for the two of us on a weeks holiday. I dont buy anything bigger as data sims expire in 30-90 days depending on the size and we dont go away often enough to make full use of a larger data sim. You can tether up to 6 gadgets to a Huawei mifi
Like woodman , I have motorhomwifi i boost , absolutely brilliant. Had ours for 4 years , using it now in France. It pulls in a wifi signal from up to 100m away, so if wifi is free at campsite Reception you will get it on your pitch , even if you can't see reception.Allows you to connect multiple devices, so if the site charges for wifi you only pay once.
We have an EE mifi which works generally fine for us. We sometimes have to move it in the caravan to get a better signal. We pay monthly from March/April till Around October then cancell the DD. Can set it up over the phone as well. Use it for phones, tablets and on occasions laptop