Wi fi dropping out to my son's shed

Hello everyone & a happy new year to you all.

I know I've broached this subject before and don't want to bore you with it again I know there may be only a few solutions to solve it. It's not practical for a variety of reasons to have a trench dug outside my back door for an external ethernet cable to be sunk into the ground & run to my son's smart tv housed in a timber shed about 4 metres from our kitchen door. Probably an expert on here has suggested Wi Fi extenders and I believe my son is going to look into this. The advice from a moderator here ( because I'm not on a package where these pods are sent FOC ) is to make sure if he buys one it should be a 'dual bandwidth' and should have more than the speed we get from the router which I'm sure is 250 mbps. What I wanted to ask you is ( speaking with an agent confused me ) our hub / router works on a dual band width ( I'm sure Jappitts & Apollo have told me this ) and regulates automatically which is the stronger signal of the two to send out? Our hub is situated at the foot of the stairwell ( this seemed the most practical place for Virgin to bring in the cable ) on one side of a 9 inch brick wall. Roughly the distance in metres for the signal to travel to my son's TV in the shed is approx 20 metres ( the length of my ethernet cable which was once used when he couldn't get a connection ) either through the wall and directly into a timber shed or along our hallway until the kitchen and out of the back door.I used to think that the signal strength was reduced at times depending on 'congestion' but the agent said this would only slow down the signal. Am I to assume that ( and we in our house don't use any gaming / laptop connectivity but  a little catch up from a V6 box occasionally ) that the reason the signal is intermitant is that from time to time the hub is redirecting the stronger signal somewhere else within the house??? You can tell I'm not tech savvy and wondered if you could explain in simple terms why this might be happening at odd times of the evening? Thank you all.If the wi Fi extender doesn't work then it's back to using the ethernet cable but leaving a window & letterbox open for it to pass through.