You want this one:
https://www.amazon.co.uk/Poynting-Om...mputers&sr=1-1
It's the more up to date version of the one in your link. This has up to 3dBi gain (the older model has 2.5). Mount it using the suckers on the inside of an upstairs window that has near line-of-sight to your nearest cell tower. You should get an immediate and probably quite dramatic improvement, but you might find that making even modest adjustments to its precise placement makes a significant difference to the signal you get.
Don't judge performance based on a speed test - access the router's admin pages and look at the network info page. You're looking for signal quality (RSRQ) to get down close to, or better than, -3.0dB and signal strength (RSRP) close to or better than -90dBm. If you can achieve these figures then your router is getting as good a signal as you can hope for.
You will almost certainly improve reception further by mounting the antenna outdoors, but this is a little more complicated than it seems; 4g signal is very susceptible to attenuation on a long cable from the antenna to the router, so you really don't want to extend the cable beyond the 5 metres supplied by the antenna manufacturer. This makes the placement of the router inside your house more difficult because it has to be indoors, but five metres from wherever your antenna is sitting outdoors. Depending on the layout of your house you're then possibly looking at taking measures to mitigate poor wifi coverage - but this is relatively straightforward.
Incidentally I'm using the B818 which is very similar to the 535; it has a couple of extra tricks but nothing worth shelling out for if you already have a decent one (and you do). Your next upgrade would be to a 5g enabled one, but they are still pricey, so hold off for a year or so.