@audimax 

That is weird. Beside the "Backup & Restore" you are also missing the "Firmware Upgrade" menu.

What both have in common is that they include means to open a file system dialog window to the local system.

Any chance you are using some uncommon type of browser or have the security settings of your browser set ultra high or your user account on your operating system is extremely limited?

Anyway, provided any of the above is the case, then you could try to open your router's configuration page with a more common browser, lesser security settings or using a different device altogether (e.g. internet browser of your smartphone) and see if you can get the two missing menu items to show up.

@woozle thanks for your quick reply

I didn't realize the missing firmware update...

Tried it on windows (Edge and Firefox) and ubuntu with (Firefox and Chormium) and it didn't change.

However I just found out that if i switch the Router vom 3G/4G to AP it looses almost all of its Settings, but the missing buttons show up.

Any Ideas on how to do a backup in 3G/4G mode?

Thanks :-)

@audimax 

I have no clue.

By "looses almost all of its Settings" do you mean that your previously customized settings for "3G/4G" mode have been set back to default after you've moved the mode switch from "3G/4G" to "AP" and then back to "3G/4G"?

@woozle 

By loosing all settings i meant i.e. the wireless settings revert back to factory defaults. After switching back to 3G/4G the settings are back again and that means i can backup via AP mode.

And i found out, why there is no menu item in 3G Mode:

In 3G/4G the ethernet port is WAN, so its remote management (which i activated). I didn't find it in the manual, but i think it's a security feature to not be able to get the conf.bin and do firmware updates remotely.

Thank you very much for your help :-)