Best I've managed with 4G+ on EE was around 430mbs down using an old banger Sony XZ Premium, I've got similar results using a Netgear Nighthawk M1 4G Router ( highly recommend these if your not into 5G), using an external Ariel on the Netgear (Panarama) didn't seem to make much difference, slightly slower on the Down but did improve the Uplink from about 85 to 150 up.
Bear in mind that if you use an External Antenna then your reducing your Mimo to 2x2 against the Internal 4x4 that most decent Routers will use.
If going for external, try and go for an Omni setup above Roof Line that will give you better Carrier Aggregation choice.
