Of course if 9dBi is not enough you could switch to the Mikrotik LHG LTE kit which has a built in 17dBi parabolic antenna for only an extra £30. Got to beat messing about with an external antenna.
Actually the LHG only provides 17dBi at 1 frequency (Band 7) at Band 3 it is closer to a claimed 15dBi (and is very variable in real world testing) while at Band 20 it is around 5dBi
Excessively high gain will cause issues with airtime sharing at the mast with other users, so what you may gain in terms of improved RSRP you will lose in performance terms when the mast is under load.
Also don't use RSSI as any marker with 4G it isnt relevant - RSRP is the key metric followed by SINR which will never be static as it is relative to RSRQ which is realtime dependant on user and loading on the cell.
As you cant change your orientation to the mast sector antenna much (unless you have a big site) you can only aim for a balanced connection. Improving SINR will generally gain you increase in bandwidth, but higher gain will not deliver that alone.
These are the metric indicators you need to look at
Bear in mind that for 4G Home - EE use 5dBi at 1800MHz only - for good reason - it ensures uniform performance for all subscribers