I have several Denon HEOS speakers, AVR, AVRX Receiver, etc. Often (but not always), Spotify lists my Google Chromecast speakers but not my Denon HEOS speakers. If I turn the phone's WiFi off and back on, it finds everything. It does this on multiple phones. Once it sees them, everything works fine.
I am on Spotify Premium Family. Using Google Pixel XL (Android 9) phones. I live in Canada.
You might need to make sure everything is up-to-date. Find out if your HEOS speakers have a firmware update which you can install. You can also try to reinstall the Spotify app on your mobile device using these instructions. Of course it goes without saying that your HEOS speakers need to be within good range of your network. If this doesn't work, we recommend factory reset your Denon speakers.
Hope it'll work! Let us know if you have further questions.