When I'm on vacation, I don't trust any WiFi network. Usually I just buy a simcard from a local provider. But with a VPN connection you should be fine and much safer. So why not use your (old) Raspberry Pi? There are a bunch of options, I went for Open VPN with PiVPN.
PiVPN is really simple to use and even on the Raspberry Pi 1 you can setup a VPN server.
Installation is as simple as
curl -L https://install.pivpn.io | bash
Configurarion is just following the steps that are mentioned during the setup.
Transfer the .ovpn file to your device after you've added some devices and that's basically it.
On mobile install the official Open VPN app from the app store, import the .opvn file and connect.