I did this a few months back. I tried a variety of equipment.
My ordinary router is an ASUS, which has a failover. It can either use the USB or an Ethernet port. It's not really that important, though, because you can just manually unplug your current modem and plug in a 4g router in its place.
Having tried a few, I particularly like the Zyxel because they had a reasonable amount of configuration, and you could set the IP on the LAN side to 192.168.0.1 which didn't clash with my normal routers settings of 192.168.1.1.
You can get a zyxel router, used, for that sort of money. TP-Link offer some alternatives, but I didn't try one of those.
There is little point in buying a fast router if you are in a poor reception area. A cheap one will perform just as well as one costing twice the price.
As to which service you buy, the best I found was 3. Their SIMs are available in the jungle, and they just plug in and work. Others, you have to put them in a phone first to register the SIM before it works.
I am still not entirely sure of all the rules for the SIM, but I think it has to be used once every six months, or it stops working altogether and in addition, most of them also lose their data allowance within a month if it is not used.