So you need to use them. Cycle them through. (Each battery has a unique serial number so you have the option of various levels of rigour regarding battery health and maintenance.)
If you are going to use a second battery in order to double your time on battery each day then that won’t be a problem.
That’s putting a positive spin on it.
That is hopefully a temporary situation.
Are you saying that the mobile network is unreliable where you are?
I think it is even more complicated than that. You are assuming that the state of the modem with respect to the mobile network makes any difference to its power consumption. I’m certainly not across what sleep modes the BM818 has and what the power consumption is in each mode and whether the sleep modes are being used at all and what conditions affect what sleep modes are possible.
That’s a lot of unknowns.
As a blackbox modem, it is difficult to know what exact factors will influence power consumption. If you had a dedicated test rig, you could presumably measure it even though the modem is a blackbox.
Fairly obviously the modem is drawing enough power 24x7 to listen for transmissions from the tower.
I think you would have to be a 3GPP guru even to start to know what the expectations should be.
Just my 2¢.