I have recently acquired Three Homefi and I did several things to improve my speeds. Admittedly I was starting from an already impressive baseline with the supplied router (40 mbps down, 15 Mbps up, a vast improvement from our old now deceased landline 2.5mbps down 0.5mbps up). These were great speeds and I was more than happy with them but felt there was more available and wanted to future proof things a bit. First I checked
https://www.cellmapper.net/map to see if the Cellular mast we have direct line of sight of on the hill overlooking the field behind our house did in fact have Three Cells (It did). I think it is uesful to know where your nearest Three Cell tower is as this may help figure out best placement for your router or external antenna. Second I purchased an external driectional antenna, an omni directional one may be better depending on your circumstances (depends if you have direct line of sight), but because we had direct line of sight to the cell tower I went directional. Third I purchased a rather expensive Netgear M1100 4G router (I got mine second hand on Ebay for half the retail price, still expensive though). With all of these upgrades I am now getting up to 120mbps down 40mbps up, obviously these are maximum speeds but my average speeds would be 100mbps down 30mbps up so I am extatic. If I look at the speed improvement I got from the baseline it was almost exactly 50% due to the better router and 50% due to the external antenna. I was a bit surprised the antenna made such a difference as I had a fulll strength signal before I attached it to the router, but I guess it improves signal quality not just strength. Of course this is all highly location dependent and things I did may not be helpful in your circumstances, I am lucky to be in a situation where I have direct line of sight to a Three Cell tower and because it is a semi-rural area with low population density the Tower is not likely to become congested. If your tower serves a large population you may find you speeds are restricted by congestion at certain times of the day.