Why would you pay an extra £50 a month for a BT line?
Plusnet, Vodafone and others have an up to 40mb line (inc line rental) for £20-25 a month.
I believe you should go 4g as your backup connection.
You should get a 30 day contract.
Mobile network is called EE.
https://shop.ee.co.uk/dongles/pay-monthly-mobile-broadband/4gee-router/details