Thanks, as I thought, your container doesn’t yet have an eth0 device that is configurable individually, as its inherited from the profile.
Setting the volatile key directly is an approach, although really those volatile keys are meant for internal usage and so their purpose can vary over time, so I would not recommend it.
Instead try overriding the profile’s etho device on your specific container as follows:
lxc config device add mycontainer eth0 nic nictype=bridged parent=routebr hwaddr=my:static:mac:address
Then if you need to change it in the future you then should be able to do:
lxc config device set mycontainer eth0 hwaddr a:new:mac:address
And if you want to revert to automatic volatile generation of MAC address you can then do:
lxc config device unset mycontainer eth0 hwaddr