Home Assistant iOS app not switching automatically from Local URL to external URL

Hello
as indicated in the title, Home Assistant iOS does not switch automatically :
° when connected to my local WIFI, the app connects with no pb
° when not connected to my local wifi
==> using a browser works fine when entering the external URL
==> opening the app, I get the message “the internet connection appears to be offline”…although I have updated the external URL parameter …If I reinitialize the app I can enter the external URL and it works fine.

Question; what do I miss to make the app switch transparently from “local” to “external”?
Thanks in advance