Fully battery powered door buzzer

Hi,

I have a garden door which I would like to open without a key. Unfortunately, there is no power supply nearby. However, it is in WIFI range and I could also get it in range of other RF protocols if needed if there is an adapter/sender from ethernet to this protocol.
Unfortunately, I did not find any self-contained device like Nuki or Homematic Keymatic which is waterproof, which I would actualy prefer as an option. Even the mechanic ones I found are not water proof unfortunately.

First question: Does anyone know a simple option here?

If not, I was thinking about using a Door Buzzer / door opener (such as https://www.amazon.de/-/en/Abus-Electric-Door-Opener-575549/dp/B00JLGC228) and connect it to a battery. For this to work I would need a relais that I can trigger from my home assistant somehow. The relais would also need to work on battery power continously. Is this even feasible, has anyone dones something like this or does anyone have an idea?
Thank in advance for any feedback!

Best regards,
Jonas