Netgear Nighthawk MR1100 drops in and out even after adding a 7dBi Gain antennae

I live 48km from Perth at Serpentine on the coast side of the Darling Ranges.. My ADSL2  connection deteriorated over the past 5 years and drifted between 0.4mbps and 1.6mbps despite numerous visits from Telstra who grumbled it was poor connectors at each node  and that I lived 4 km from the exchange. As I had no problems with my Telstra mobile phone connection on my i phone 8 (although it was only 2 bars)  I would give mobile broadband a go with Telstra. My Nighthawk MR1100 router arrived on 28 December and is performing even worse drifting in and out of connection. I did an Ookla speed test and it sits around 0.4mbps...worse than my ADSL. I have had the router checked by Telstra at a Telstra shop and nothing wrong with it....was reading 38mbps. They suggested  I purchase an antennae from JCar.  I have attached a $69 Digitech 7dBi Gain 3G &4G antennae and initially it registered 28mbps but  an hour later and it is down to 0.1mbps. Has anyone got any suggestions as we can't even do home banking and I am trying to run a business from home.Could someone also advise me how I can do a speed test through Netgear?