Not sure what you mean John - he's just looking for a more powerful 4G antenna to get better bandwidth, not turning into a radio transmitter, which as you say would attract a license. These are all within the existing mobile laws.
Plenty available, for example these ones with an external antenna - I'm putting one on our static caravan to get a better EE 4G signal!
https://www.solwise.co.uk/3g-routers.htm