Did you try disassociating the current floatingip and then reassociating with the new Port that you created from a subnet connected to the new router.
Just to make sure if this is a cleanup issue.
Also can you check if the router-namespace has the correct DNAT rules for the floatingip and the Private IP of the VM ( instance ) port.
Check if you ping reaches the VM by sniffing the external gateway port on the router namespace.
See if it hits the DNAt rule.
Did you try disassociating the current floatingip and then reassociating with the new Port that you created from a subnet connected to the new router.
Just to make sure if this is a cleanup issue.
Also can you check if the router-namespace has the correct DNAT rules for the floatingip and the Private IP of the VM ( instance ) port.
Check if you ping reaches the VM by sniffing the external gateway port on the router namespace.
See if it hits the DNAt rule.