For beginning welcome to PC Expert forum,
lately "fashion" is that ISP installs / sets devices with custom fw version in difference of some past times,
mostly they do it in order to hide this VoIP account settings or customize to their network (ACS server, TR069, remote config).
In particular your OTE version of Speedport is still good, the more options are open and available to user.
You have to pay attention to one detail, sometimes they deliberately set TR069 in correlation with the VoIP service,
In other words, if you kill TR069 may lose your VoIp (telephone) service.
Regarding the firmware, it is normal that there is a control mechanism that prevents insertion of inadequate versions,
That's why "do not pass" our (T-Kom HR) or DE versions.
What you can do:
a) figure out how to log on as an administrator, not as an ordinary user vulgaris
try to figure out how to stop and turn off the TR069 service
b) as you've already mentioned, ask from ISP to change device (Speedport) config and set Internet connection in Bridge mode,
behind you will connect your WAN (Wireless) Router
That's all, regards