The Serial intefaces is already available for the lxc containter and needed to be add as a filesystem node in the guest.
Before you create the node you need to get the MAJOR and MINOR Number which can seen in the /dev directory with the ls -al | grep ttyS in the router root shell.
USER
crw-rw-rw- 1 root root 4, 64 Sep 30 15:59 /dev/ttyS0
crw-rw-rw- 1 root root 4, 65 Jan 1 1970 /dev/ttyS1
crw-rw-rw- 1 root root 4, 66 Jan 1 1970 /dev/ttyS2
crw-rw-rw- 1 root root 4, 67 Jan 1 1970 /dev/ttyS3
crw-rw-rw- 1 root root 4, 68 Jan 1 1970 /dev/ttyS4
crw-rw-rw- 1 root root 4, 69 Jan 1 1970 /dev/ttyS5
crw-rw-rw- 1 root root 4, 70 Jan 1 1970 /dev/ttyS6
crw-rw-rw- 1 root root 4, 71 Jan 1 1970 /dev/ttyS7
then in the lxc container you can create the the device with the commands
ROOT
mknod /dev/ttyHostSerial0 c 4 64
mknod /dev/ttyHostSerial1 c 4 65
and then access the interface with microcom for example
ROOT
microcom -s 115200 -p /dev/ttyHostSerial0
microcom -s 115200 -p /dev/ttyHostSerial1