Hi
Check this post for similar configuration: [Solved] ModBus and cheap 4 relay module (D18994) - #5 by stabilis
In the linked example, numerical value 256 (0x0100) is written when command ON is received. With command OFF numerical value 512 (0x0200) is written. Note that example is write only, it does not read the value from modbus. You can make it write with some more configuration.
See separate index for reading and writing and extended item configuration format for more info.
Btw, I noticed that you are now reading coils, but in your post you talk about registers. Which one is it?