Please change the mode of the serial Interface you want to use to “none”
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.
then in the lxc container you can create the the device with the commands
and then access the interface with microcom for example