Ejjoman, the issue was indeed that I'm using a Jolla C so rootfs is already mounted by default apparently. I could mount it to /mnt/rootfs and chroot in doing as you said though. However, I can't seem to get any Internet on the phone in recovery mode after doing the ip forwarding/iptables/route manipulation in the guide you cited. I did everything as explained (Linux computer), no error, but yet no connection. :/

I'm clueless. I did everything as mentioned in the guide, the 10.42.66.66 (Jolla C Recovery shell) and 10.42.66.67 (computer telnet interface) IP correspond to what is written in the guide as well, got no error, but I cannot get Internet on the phone.