Is it a Raspbian image or a piaware image?
This guide is for the piaware image: How to Install and Configure Piaware 5.0 SD card image - Quickstart Guide
If you have the piaware-config correct with the serials it should work.
Do a reboot just to make sure the configuration is applied.
You can share the logs for dump1090-fa and dump978-fa via pastebin.com, see the commands to get the logs here:
Debug commands · wiedehopf/adsb-wiki Wiki · GitHub