The receivers only output for subwoofer is coaxial

No, the AVR you have is also RCA output.

https://imgur.com/a/ZihiQlI

So you'll run a single RCA to RCA cable from the AVR to the subwoofer.

https://www.amazon.com/Mediabridge-ULTRA-Subwoofer-Cable-Feet/dp/B003FVYXY0

That is all you need to connect the subwoofer to the AVR.