Les Gibson, no landline , broadband or wifi.
In which case I would say that your only option is to ask your employer for a sheltered area outside the industrial unit on some pretext, e.g. a smokers shelter
Have a look at the Ofcom checker for indoor coverage. If another network is better then pick up a PAYG SIM to try. If that works it's probably more economical to switch networks than mess around with dodgy boosters. Or if your phone supports dual SIM, divert all calls to the second SIM while at work.
In which case I'm surprised you do any business without any broadband or landline. Bite the bullet and get broadband installed, yes it's a cost on the business but having poor or no Comms must surely be a bigger cost
I'm inclined to agree with @Les-Gibson, you don't have many options available to you...
Signal booster looks quite expensive and there's no guarantee that they'll work. If you have a Wi-Fi Calling compatible handset depending on the size if the unit broadband boosters for Wi-Fi are available (BT call it 'Complete Wifi') might be your best option.
You can check coverage for other networks and see if they can offer you better indoor coverage. Vodafone has typically, where EE have struggled indoors, been a little bit better but if you're in a steel/corrugated unit then you might not have much luck with any network but it's worth a try!
Many phones are dual SIM, you can have 2 lines active so if line 1 is unavailable then you can divert it to line 2.