SIM trouble in the UK - HELP!

I purchased my phone in April but have just started trying to use it. I'm based in London UK but for reasons at the time, had the phone delivered to Canada. Either way, the manufacturer assured me it was unlocked and listed the available networks:

4G: FDD-LTE : BAND1/BAND3(2100/1800MHz)
TDD-LTE : BAND41 2496-2690MHz
2G: GSM 850/900/1800/1900MHz
3G: WCDMA 850 / 900 / 1900 / 2100MHz

My phone specs are:

Model Number: LEO C
Android Version 4.4.2
MIUI Version: MIUI-4.6.20
Basband Version: 3.4.0-g26e4aff-00865-g1dabe2b
Build number: KVT49L


I've tried 2 different SIMs from BT (connecting to the EE network) and I've had intermittent service -- most times having no service at all. BT has assured me that it's not their network (or the SIM) and that they are compatible with the FDD-LTE specs provided by the manufacturer.

I'm wondering if anyone else has had this problem or can recommend a network company that will work on my phone. It's super frustrating having an Xiaomi Mi4 and not being able to use it. After speaking with BT 6 times in the last 2 weeks, and not getting ANYWHERE, I'm feeling at my wits end about this!

Thanks!