If you want to check broadband or mobile broadband coverage for anyone use the Ofcom Mobile and Broadband Checker website. If there isn't superfast of ultrafast fibre available then there are a couple of other options. Whatever they need, we can quote.
You can check the coverage maps on the Ofcom website to see who gives best signal at the customer's address but bear in mind it's not 100% accurate, can vary, and can be a lot worse indoors vs outdoors or downstairs vs upstairs. They also need to make sure they get a business contract with mobile providers as they are very funny about people using home accounts for business and will just cut you off.
I will let you know what difference my external antenna makes when it's fitted and pointing at my nearest 5G mast!
You can look at a leased line but this is the most expensive option. There is an install fee to have the cable installed to the customers premises in the first place, and this can be costly depending on how many roads and paths need to be dug up! And there's a high monthly cost too. There may be grants available towards the install which we can check for them.
Alternatively they can move to somewhere with better broadband. There may be other options in the near future such as satellite broadband from Starlink but this presents its own problems as the distances involved in transmitting the data into space and back can have a detrimental effect on things such as VOIP calls.