I was a telenet before, but we changed a long time ago, and i just found the modem somewhere

The telenet modem is a ch6643e model, which has a nice wifi and gigabit ethernet. I was planning to use it at the last floor of the house but i looks like there is no way to configure anything since you have to do it from the mijntelenet interface.
Is there a way to flash it or upgrade firmware using usb, or install OpenWRT or whatever way to use it ? For now it is using it's range (192.168.0.x) and i would like to disable DHCP to use it as a switch only.
I've seen in some forum that you cannot connect using telnet but i could not acheive that; i port scanned the device and only 53 and 80 are open. Which is weird, because when i send commands to port 80, it never answers..
Did anyone could do anything with that modem ? Or have some clue how to re-use it ?
Thanks

Xavier.