I bought used via ebay from a company reselling ex office equipment. There are various brands available if you search on 4G modem. It's really just a standard modem-router with a 4G card inside: white oblong box with a pair of antenna on the back, network cable sockets on the back, mains powered, plus a slot on the side for a SIM card.
Interestingly EE is the only network that gives truly usable 4G speeds in my house. I've tried them all.
I also have a battery powered version, i.e. a mi-fi device, which doesn't work quite as well as the unit with proper antenna described above, but is OK in a pinch. Again I paid about £30 for a used example.