I thought the harmony hub only worked on the same network, but I turned my wifi of on the phone and it still worked, so that is definitely an option for you
Thanks for the suggestions.
As an update I bought a broadlink RM mini 3 as it sounded like it'd do just what I wanted, the reviews were good and it appeared easy to set up.
Unfortunately if it doesn't connect to the wifi on it's auto setup there is very limited help in trying to get it to connect. I've tried and tried, so I'm on the look out for a replacement, and am off to google the harmony hub.
You could also look into sofabaton, they are a new product and are still being produced unlike the harmony. They have there own forum, if Y were you I would post there and ask if it will work for you.
Let us know what you find out
Although new Harmony products are not available it appears there is still an active support section for the range within Logitech as I have just had a new firmware update for my Harmony Ultimate One model.
I have had a message back from an eBay seller of the harmony goods, he has loads for sale. He says it won't work as the phone and hub need to be on the same network and the remote obviously wouldn't work, as it is RF. The remote didn't matter, so the hunt continues.
He's wrong, as two of us have already confirmed in earlier posts.
The Harmony can be set up so that IR codes are sent from the remote or from the Hub. In your scenario the remote would work but isn't necessary. You could control the Hub directly from the app, so you don't need a remote, just the Hub.
I have a Harmony Hub and can also confirm the app on my phone works fine over a mobile connection rather than wifi - it uses a cloud connection instead of local wifi.