Hello @KGrev ,
unfortunately there is no multi point L2TPv3 feature as far as I know.
However, replicating a VLAN L2 over cellular was prohibitive years ago for performance and costs.
Now with 50 GB per mounth contracts can be considered.
>> I'm needing to span a flat vlan across multiple points with Cisco IR809G's at each location (over cellular cloud)
Try to check if you can use NAT instead or a routed solution that would be far better from performance point of view given also the type of routers involved.
Hope to help
Giuseppe