Question regarding chinese voltmeter ammeter module
The voltmeter/ammeter needs a voltage in a certain range to operate properly. That's the thin black and red wires.
Usually claimed to be something like 4-30V but you might want to be a bit more conservative.
Your two 'or' diodes should be upstream of the regulator module, and should be applied to the POSITIVE input, not the negative. So from that junction you have 24V - 0.7 or so if powered by the 24V supply, and whatever your external input is minus a diode drop (whichever is higher at the moment). If that fills the specification of your meter module you are fine.
Note that the current meter measures via the return current lead. The two black leads (thick and thin) are directly internally connected.



simulate this circuit &ndash; Schematic created using CircuitLab
Now, if you REQUIRE a common between input DC ground and output ground then you have a problem (that an external supply or battery will not solve) because the modules measure current on the return side. High-side current measurement would cost considerably more money than the $0.01 op-amp they use.