Two isolated ground from a single ground terminal

[Adhith]

Hello everyone...
 I'm using a 4S  18650 battery pack and its BMS board in one of my project. I'm using a constant voltage buck boost converter to boost the voltage from 12v to 18v and then using a CC/CV buck converter to buck the voltage to the required voltage of 16.8 for the battery pack. The reason I'm using two of these modules is to due to the lack of availability of an appropriate CC/CV buck boost converter. Anyhow the thing works fine since i have tested it with led strips and works as expected.
The problem starts when i hook the output of the CC/CV buck module to the P- of the BMS (battery management system) Since the ground terminal is common for the P- and  CC/CV buck converter input (output ground of CV buck boost converter) and output. So its clear that the resistance for the constant current function is bypassed and thus the constant current function doesn't work.
So what I'm looking for is a solution to separate these two grounds from each other. What might be the possible solution.. apart from replacing the entire modules since the project is almost completed and all wiring had done.
I have attached the pic of the CC/CV buck module and the schematics of the thing which I'm trying to do below
any help would be appreciated...