Glad to hear your environment is working as expected.

I tried replicating the issue with DO's default Debian droplet and it went fine ... adopting FS's repo key was a non-issue. This got me thinking whether the IP pool your server is bound to may have been previously blocked (e.g https://lists.freeswitch.org/pipermail/freeswitch-users/2019-April/131716.html).

Everything should work out of the box if the repository is up and accessible as well as if the key is trusted; the latter can be tested by running:

apt-key list

You should see a record reading "FreeSWITCH Packaging Key" or so.

-C