External Antennas are not directly related to Download Speeds, the External Antennas are for improved Signal Quality and Stability in poor Coverage Areas which in turn may improve DL Quality, in fact they can sometimes have the opposite effect in reducing Download Speeds (its to do with the fact the Router is potentially 4x4 MiMo whereas the External Aerial is only 2x2 MiMo).
Before you spend money, do some easy Tests -
Check your Post Code Area on EE's Coverage Map Checker, what results are you getting in terms of 4G and or 5G. ( https://coverage.ee.co.uk/coverage/ee check for both Coverage and Status)
Try placing the Router in various spots around your property, near windows and high up where possible, Reboot the Router and leave the Router in each position for at least 15mins before testing and moving to the next spot.
When checking Speed Tests, if possible, use an Ethernet Cable connection to the Router, if your using WiFi place the Testing Device around 2 meters from the Router.
Try the EE Router Sim in a Phone, and test around the same spots in the House and outside, Garden etc.
What is the Router Model Number?, most Routers will have a Status Page within the Admin Settings area, this should give you some Signal Quality Info, like - Signal RSRP, SNIR, Freq or Band Info, CELL ID or EnBID Mast number etc, post up your findings. (Do not post any personal info like Phone or IMEI Numbers)
Regards Chris.
