Hello,
yes this method works, but... I have tried upgrading another device today a this is the result...
Before upgrade I turned on "Enable remote HTTP access", "Enable remote HTTPS access" and "Enable remote SSH". I am sure these settings were turned on because I started upgrade remotely via public IP HTTP WebUI. I was also able to connect via VPN.
After upgrade I am no longer able to acces HTTP/HTTPS/SSH nor VPN remotely and SIM card doesn't accept SMS (I think it's data SIM only). But the device is on - it response on ping! So now I have to go there localy and turn everything back on. I think there must be something wrong during the upgrade procedure.
EDIT:
After physical restart is remote HTTP access working! So after upgrade there must be something wrong on first boot.