Try turning off the smart wireless functionality of the hub and trying different channels manully to see if that improves things. Despite the advertising hype, the wifi range of the hub will be fairly poor as it has internal aerials. A third party router with external aerials will give better coverage. Alternatively, you could consider wifi hotspots such as these https://www.shop.bt.com/products/bt-mini-wi-fi-home-hotspot-600-multi-kit-084289-BVFH.html
Before doing anything with the wireless speeds you need to check that your broadband is working as it should and is delivering the correct speed when connected by Ethernet.
Can you run this checker and post back a screen shot of the results. Make sure you delete your phone number before posting.
and use this speed tester then scroll down the page and carry out the further diagnostics and post back a screen shot of all the results including your IP profile for up and down. This test must be done with a wired connection.
I expect you have the Smart Hub, (Homehub 6) can you go to hub manager then advanced then technical log and post the stats. http://bthomehub.home/