With most Telekom.de plans you can opt-in to StreamOn Video, which lets you stream videos for free without using data plan.
That port is only for video input, the USB-A port should work though.
Also, if it’s helpful, in the hidden (default) Android settings there are options to turn on Hotspot mode, so you can make a hotspot out of your PicoPix Max, connect it via a USB-Ethernet adaptor to your hotel network by a LAN cable, and enjoy wifi on all your devices plus the projector itself.
And lastly one more suggestion – you can use Miracast (the “Android” option under Mirroring). This also uses WiFi but it connects directly, doesn’t need access to a network. Theoretically you can play something on your phone/tablet/laptop and mirror it to the picopix max. Won’t work for iPhone though.