Ring Video Doorbell 2 Poor Wifi

I've just got & installed a Ring 2. I have Virgin broadband (allegedly lightning fast!) & did the connection test during set up outside my door. The test said I had a good connection. It's now been mounted oustide. I'd noticed it was very slow to load the video on my phone so checked the device health which is now saying I havea poor connection. I've checked my down & upload speeds which are 100 & 20 resepectively so way more than adequate. I've no idea why it's doing this - our wifi stretches to a shed at the bottom of our garden. Please, please tell me there's a way to fix this, will be raging if I have to return it ....... Thanks for any help in advance :-)