I bought that device on Ebay, but I can not use it, because it only works with German Telecom (Deutsche Telekom). In Hungary it does not work. There is an LTE sim slot built in it, and I would like to use it with OpenWrt. There is a dsl connection too, and I would like to use it as a WAN port to connect it to another router. Please build a firmware for it, because it is not supported yet. And sorry if my English is bad, it is because I am from Hungary.
Sorry but it's not that easy. You have to first identify what SoC it is using (assuming it's a relatively modern device), then you can try to find a similar device, compare the .dts and .dtsi files, then try to make some minor modifications to those and get it to work. Anyway, assuming you can get OpenWrt installed, you can try using ModemManager and luci-proto-modemmanager to get the LTE modem working quite easily.
I'm not sure whether that would help. It seems like the orginal firmware can be downloaded for this router now:
Specifically at Hybrid Quellcode zur Firmware. That's a pretty big package and also seems to include at least rudimentary build instructions.
Unfortunately, I don't have the time or knowledge to do get into this -- I rather bought a new router to have at least a bit of up to date security, as the Speedport Hybrid firmware hasn't been updated for some time anymore. But maybe it helps s.b. to rescue some of these devices. They can still be bought and they can also still be used with the T-Com subscriptions.