Can't use mobile data on non-default SIM, even if inactive when the second SIM is open call

I'm using OnePlus 7T with dual SIM. SIM #1 is my default SIM, and SIM #2 is a data SIM which I'm using only for mobile data. Obviously, I've defined SIM #1 to be the default SIM for calls and SMS, and SIM #2 to be the default SIM for data. When I'm in an active call on SIM #1, SIM #2 is automatically blocked. However, instead of using SIM #1 for data, the phone is simply not getting any mobile data at all. Any ideas on tackling this problem? Thanks.