You should get your hands on an OBDLINK LX from obdlink.com. I used it and an old android device to get the mileage from an R running motronic 4.3.
I tried using the generic OBD ELM units, they never worked for me. It's possible that like you said, Fenix 5.2 will not communicate to anything other than a Volvo System Tester or a blinking code tool. I wonder if it works with VOL-FCR?