Are you sure that the sub output on the AVR is a digital coaxial? That would be very unusual indeed. I'd expect that it is a low-level analogue output.
If it is, then you can plug this into either the left or right on the sub. (you could send one sub out to L and the other to R, but it really wouldn't do anything for you!)
Yep, that's fine to use as I said. (the word 'line' on their sub gives it away). Glad it was that - I'd never heard of a system with digital output to sub!
Note that on your sub, there may be an auto standby feature. This is where the sub will turn off if it doesn't detect an audio input.
There's a chance that this input detector is only on one input - left or right. So it might not trigger if you use the wrong one.
Likely the amp has a single RCA-type connector Out. You can feed that to the RCA-type RIGHT connector on the Subwoofer. It is VERY unlikely that the amp and a Digital Coaxial out. It is far more likely that the Sub Out is analog and is the Sum of the Left and Right Channels. Inside the Subwoofer, the Left/Right inputs are summed together into a MONO signal anyway.
So, I suspect, if you look in the owner's manual for the Subwoofer, you will find that if your source is already a Mono signal, you simply plug it into the Right channel of the Sub.