This issue is more likely to occur when using a ZCC, and especially when the WAN link is a VPN tunnel.
In this scenario, site connectivity appears to be healthy, and the Zerto GUI may not present any errors or warnings in regards to site connectivity.
Regardless, VPGs will never be able to begin syncing because of packet fragmentation resulting from MTU overhead somewhere on the WAN link. Therefore, any sync progress will never advance beyond 0%.
The solution is to determine the actual MTU of the WAN link and then set the VRA NICs to the correct value.
The default MTU for VRAs is 1492 bytes, therefore if the WAN link has an MTU that is less than 1492, fragmentation will occur and this issue will result.
Zerto support can assist using VRA ping tests to determine the actual MTU if the value is unknown.
Zerto support is also needed to persistently reconfigure the VRA MTU values.