Depending on how they wired your solar, the Battery Disconnect Switch will not help you.
The solar wiring for mine goes directly to the 4 gauge wire leading to the battery via a 30A inline red fuse.
You will need to trace the wiring.
As for freezing temperatures, you can draw FROM the lithium batteries at freezing temperatures.
If you charge them at freezing temperatures you will destroy them. Depending on your batteries they may have an internal BMS (battety monitor system) that will prevent them from being charged in freezing temperatures. You will need to check with the manufacturer.
Having said that, in cold weather OR if I'm on shore power I turn off the MPPT. If freezing temperatures are expected I turn off the converter circuit breaker (charges the battery).
I installed battery heaters for using the batteries in freezing temperatures.
Good article about lithium batteries
How to find happiness with lithium batteries