ssh with Termius from iPhone over hotspot to Ubuntu robot?

I would like to be able to ssh into my RPi3B+ based robot (running Ubuntu 20.04 64-bit Server) when away from my home WiFi.

I have configured the bot to recognize either my home network or my iPhone hotspot network.

/etc/netplan/50-cloud-init.yaml:

network:
    ethernets:
        eth0:
            dhcp4: true
            optional: true
    version: 2
    wifis:
        wlan0:
            access-points:
                SSIDHOME:
                    password: pwhome
                SSIDHOTSPT:
                    password: pwhotspot
            dhcp4: true
            optional: false

I see that the bot successfully connects to the hotspot.

Launching Termius on the iPhone, I created a host with the hostname of the bot - "ROSbot".

Clicking on the bot in Termius:Hosts shows "(ROSbot:22) Username / Select Identity" dialog, to which I select the desired identity (username/password) on the bot.

Termius reports:

Connection could not be established:
Connecting...
Disconnected
Connection closed with error: unknown node or service

Looking in the /var/log/auth.log there are two sshd entries:

Oct  3 10:24:56 ROSbot sshd[1921]: Server listening on 0.0.0.0 port 22.
Oct  3 10:24:56 ROSbot sshd[1921]: Server listening on :: port 22.

but no successful or failed login attempts are listed like when I remote in when the bot is connected via my home WiFi:

Oct  3 10:41:48 ROSbot sshd[2319]: Accepted password for pi from XX.0.0.XXX port XYZ ssh2
Oct  3 10:41:48 ROSbot sshd[2319]: pam_unix(sshd:session): session opened for user pi by (uid=0)

Any ideas how to debug what is happening and get this to work?