I am running HA in a docker container and using duckdns. I can reach it most of the time but randomly both browser and companion app fails to connect. In browser i get error "Unable to fetch auth providers. https://myhost.duckdns.org:8123/?auth_callback=1". Then after a random amount of time it works again for a while and repeats.
#Unstable remote access
storm hornet
placid pollen
what ever issue you get in the browser needs to be fixed for the app to work, depending on your network setup this troubleshooting page may be of help https://companion.home-assistant.io/docs/troubleshooting/networking