Hello,
Thank you for reaching out!
At the moment, the only solution could be to use cell locking to achieve this. This procedure can be found here.
Select the B3 or B1 cell in question, and lock to it. The device will still be able to aggregate with B20 as a secondary band.
However, if the device does not aggregate the bands automatically, it's quite likely, that the available cells simply cannot provide a good enough signal to maintain a stable connection.
Let me know if any more help is needed!
Best regards,
DaumantasG