Getting a root shell on an old KPN Experia Wifi (CVE-2021-38703)

The KPN Experia Wifi was sold by KPN/Telfort in The Netherlands as a WiFi amplifier/extender, for those households or businesses that need that extra WiFi oomph. On mine, running an old firmware version, I managed to gain a root shell. KPN asked that I state clearly here that the vulnerability has been fixed since, and updated devices are not affected.