Your network setup seems correct, enp0s29u1u1 is the ip over usb interface and it is correctly configured. You should use command "telnet 192.168.2.15 2323", there's no colon in the command.
telnet 192.168.2.15 2323 Trying 192.168.2.15... Connected to 192.168.2.15. Escape character is '^]'. Welcome to the Mer/SailfishOS Boat loader debug init system. Log so far is in /init.log To make post-switch_root halt before starting systemd, perform: touch /init_enter_debug2 (When run post-switch_root, telnet is on port 2323, not 23) sh-3.2#