Hi Wizard,
I hope I am not too late ...
The process you are going through seems very frustrating, complicated and too some extent unnecessary. Although the Huawei router you have been recommended is okay, what you are experiencing with it is very typical. Unfortunately adding yagi antenna's to the outside of the building isn't going to improve matters significantly, you'll probably lose all the antenna gain in the cable run.
If you can't get 4G and you have some 3G coverage, I would suggest the better option is a WiBE (The indoor unit retails for approx £348, so is more expensive than the B593 but you don't need to worry about the cost of external antenna's, buying the right ones or installation).
The WiBE router, includes four high gain antennas built into the unit itself, hence why there's no need for external antennas. The antenna's are also self optimising as the unit scans for the fastest connection and focusses the antenna gain in that direction. So if your network provider decides to move tower locations, you don't need to worry about realigning directional antennas. (A technology video for WiBE can be found on youtube here ->
http://rgxd.at/v1).
If you have got some indoor 3G coverage, then WiBE should be sufficient. If you have no indoor coverage but 3G signal outside, then WiBE Extreme would be the better option. (This customer story maybe relevant from a WiBE customer in Cornwall who's also on EE ->
http://rgxd.at/tol)
Please be aware that I do work for the manufacturer of WiBE (RangeXD is a sister company to Deltenna and handles sales and marketing of the WiBE portfolio ->
http://rangexd.com), so you probably won't consider me independent, but I did want to point out an alternative option. Please do take a look at one of our stockists, ruralBroadband.co.uk (tel:01485572253) who are offering a free trial of WiBE at the moment, so you do have a no risk option to benchmark performance. They also stock the B593 so they will also be to provide guidance on how to improve the performance of the B593.
I'm happy to be contacted if you have any questions. Just drop an email to
info@rangexd.com.