I’m making some assumptions, namely that you’re using an unprivileged LXC container and the mount point is a bind mount.
Unprivileged LXC shift user ID numbers so that an escape won’t result in root access to the host. The root user (uid 0) in the container is actually uid 100000 from the perspective of the Proxmox host.
What I usually do is set ownership of my bind mounts to that high-numbered ID (so something like chown -R 100000:100000 /path/to/bind/mount) from Proxmox. Then the root user in the container will be able to set whatever permissions you need directly.