adding an LXC with a different subnet

[krikey]

I have PVE 8.2.x running with the following configuration:

iface eno1 inet manual

auto vmbr0
iface vmbr0 inet static
        address x.x.x.13/24
        gateway x.x.x.1
        bridge-ports eno1
        bridge-stp off
        bridge-fd 0

I'd like my LXC to use y.y.y.63/24, gateway y.y.y.1

The gateway is upstream and not within my control.

I've tried adding the following to PVE but still not able to reach the WAN from the LXC

iptables -t nat -A POSTROUTING -s y.y.y.63/24 -o vmbr0 -j MASQUERADE

Here's the .conf file for the LXC

arch: amd64
cores: 1
features: nesting=1
hostname: test
memory: 512
net0: name=eth0,bridge=vmbr0,gw=y.y.y.1,hwaddr=11:22:33:44:55:66,ip=y.y.y.63/24,type=veth
ostype: ubuntu
rootfs: local-lvm:vm-100-disk-0,size=8G
swap: 512
unprivileged: 1

Can anyone assist with this?