m5200 speed and signal

Hello everyone, 

I bought the m5200 recently in order to avoid getting into another broadband contract. We live in NE London and I bought a Three unlimited 5g sim. On the three coverage map it should I am right on the edge of 5g coverage, (the shading comes stops directly in front of our house, across the street is within coverage.) 4g coverage is everywhere around us. 

As soon as I opened the box I updated to the latest firmware and connected my macbook but can only get a download speed of 7-10 mbps.  No matter where I move in the house or in the garden. When I check the status it says: Network status: LTE, RSRP -111dbm, PLMN:23420, Band: LTE B1

At this point I am pretty discouraged because I paid £300 for the sim that lasts until next year but the speeds are terrible. Do I need an antenna and if so, which one might you recommend? Any other advice for making this work?  Thanks