Good 3G poor 4G - antenna?

After hearing so many horror stories about mobile broadband I avoided it testing it properly until now but after deciding to investigate it I am pleasantly surprised with the results (Three's network). I bought a Huawei B593s-22 and put it in the attic of my house, 3rd story in a rural area. 3G signal is very strong, I'm seeing 4 bars on the router interface and stats say EC/io of -2dBm, speed tests reports 20-30mbps downstream 5mbps upstream off peak and range between 6-12mbps down off peak. Real world usage doesn't seem to quite match speed test results but even at peak times I'm getting consistent streaming rates from Youtube and Netflix that make wireless a very attractive prospect over my DSL line.

LTE is a slightly different story. I'm basing my information on taking directional information from my phone and matching it with Siteviewer and Google Earth for distance and LOS, hopefully it's correct. There are 2 sites the router connects to: the first site is 17km away with a tree likely interfering with LOS and the RSRQ RSRP SINR stats are -12dB -88dBm 1dB. The 2nd site 7km away and the treeline is interfering with LOS - Google Earth says there is just about LOS for this site over the ground - the tower is at a lower level to my router in the attic hence the tree issue, stats for this site are -7dBr -104dBm 7dB for RSRQ RSRP SINR respectively.

The router reports only 2 bars of signal for 4G while 3G gives full signal, I'm not sure why 800MHz 4G performance doesn't improve on 900MHz 3G performance (I'm assuming given the distance from the mast higher bands for both are not in play - though would 30mbps 3G imply diversity using higher and lower bands in different channels?) but it could be that the router is connecting to a different 3G tower with a better LOS that my router cannot use LTE with. There are many different cell ids reported for 3G and I haven't worked out which is which so I'm not sure what the answer is. It's not really possible to compare the router's service with my phone as at ground level I only rarely get 4G and then it's for a few seconds before reverting to 3G.

Peak rate for the 7km site never matches 3G performance (max I've seen is 18mbps down but it rarely exceeds 12mbps at any time occasionally the upstream goes to dial up modem speeds) and I'm wondering which directional antenna if any would improve the LTE service and how much improvement could I expect using it using LTE? I know the answer involves speculation so feel free to speculate! I'm happy to continue using 3G given it's impressive performance even off peak but I would like to investigate what could be done with the LTE side of things, an improvement over the 3G is a tantalising prospect.