I have been googling this subject because at some locations I need to go outside of boat ,ok with warm sunshine ,to get any internet or calls.
There are booster for cell phones but ofcom made some illegal ,so they are for sale online but you need to check it's the legal type .if advertised as open to any network it's the naughty type and can cause interference. Legal ones are setup for the network u use.
I dont have one so not sure there any good ,and my info is just what i picked up with 1/2 hours googling