Sweet, that was what I was after:
opkg update
opkg install screen
root@Teltonika-RUT955:~# screen /dev/rs232
screen: can't load library 'libncurses.so.5'
ln -s /usr/lib/libncurses.so /usr/lib/libncurses.so.5
now I can do
ssh -t root@10.7.0.7 screen /dev/rs232 9600
and I get access to the serial port
Thanks!