But since your device is a DSL modem/router, with no regular WAN port, dumbap is pretty much the only way to use it, until you replace the Easybox with it.
I would like to connect the router via usb tethering with my smartphone, if that would be possible and then use it as a wireless access point, because i have no internet in my second apartment only my datenflat
in that case you temp have to set up the Frtzbox as a dumb AP (link provided earlier, only do step 3 & 4), so it can coexist with the Easybox,
In addition set a static IP using the commands:
uci set network.lan.ipaddr='192.168.1.x'
uci commit
/etc/init.d/network restart
where x is a digit 2-253, but outside the DHCPs scope of your Easybox.
Once that's done you will probably be able to ping 8.8.8.8, or any other IP, but not www.google.com, etc.
If that's the case, do
echo "nameserver 8.8.8.8" > /etc/resolv.conf
Now you'll be able to DL the modules required for thethering.
When that's done, you will have to convert it back to a router again, with the WAN port mapped to your phone.
Another option is to DL all the modules to you laptop, then send them over to the Fritzbox using winscp, but then you have to know which packages you need.