You probably cant through EE now, as I believe EE only offer the External Antenna Service with a new Router Purchase or Contract.
You culd still do something your self, even if it means sourcing the Antenna Hardware then either DIY or get an Antenna Installer to help you.
In order to move forward, why do you feel that you need an External Antenna, what Make and Model is the 5G Router, what speeds are you getting at present, what 4/5G Signals are you getting in your location.
