No lock to 4G on MiFi

[#1]

I am using Plusnet SIM in Huawei E5372 MiFi, always located at my home and always for data only. The Plusnet SIM has replaced an EE SIM previously used in same device for many months without a problem. The firmware on this device allows option selection either for the connection to be  locked to 4G only or else to be preferably via 4G but with auto fallback to 3G. The problem I have encountered is that when locked to the 4G only option, no connection at all can be made. However, perversely, when the preferably 4G with 3G fallback option is selected then a connection is achieved which quickly becomes  and so far stays via 4G. The problem created by this is that as I live in a less than perfect 4G signal area it is likely that at some point the device will drop back the connection to 3G and experience has taught me that data speed is still greater with a weaker 4G signal than with a stronger 3G signal. Plusnet can offer no explanation of why the EE SIMs and Plusnet SIMs work differently in this respect and are sending out a replacement SIM so as to check if the existing SIM is faulty. Can anyone suggest an alternative cause?    

[#2]

Seems there are no theories on this problem. However it would be good if someone with an Android device would try locking their phone to 4G instead of 4G/3G (it is reversible!) to see if they have the same problem. I do know that non jailbroken iOS devices do not have this option. 

[#3]

The e5372 has a range of frequencies (varies by exact model worldwide) that it will receive the 4G signal on.  The frequency bands that the Plusnet SIM can receive in your location maybe the issue. Different providers will make a difference but so will the bands that EE allow Plusnet to use. EE keep some bands to themselves for their high paying contract customers.

* Huawei E5372Ts-32: Band 1/3/7/8/20 (800/900/1800/2100/2600MHz)   (3560mAh battery)

* Huawei E5372s-32:  FDD 800/900/1800/2100/2600MHz  (1780mAh Battery)

* Huawei E5372s-601: Band 3/20/38/40 (FDD 1800/2600MHz & TDD 2300/2600MHz)

[#4]

Many thanks for your response but I do not think this can be the cause of the problem as the EE SIM I have been using without problem in the same device was only PAYG and furthermore I believe the only EE mast I can connect to (in my fairly remote location) transmits at 1800Mhz. Surely if  this was the problem then I would never be able to connect on 4G with the Plusnet SIM and yet I can and permanently but only when using the device's 4G with 3G fallback option, not the 4G only option.

[#5]

Let's hope the SIM is faulty

[#6]

Replacement SIM still works in same way as original SIM. Clearly there is something in the way EE treats Plusnet traffic which causes this. All a bit disturbing!