Synology has released Synology Router Manger (SRM)1.3.1 that expands the mesh capabilities of the RT6600ax to work with both RT2600ac and the MR2200ac models in mesh.
There are also some solid features in the upgrade, including VLAN support. You can check out in the release notes here.
SRM is the operating system designed for Synology networking products. Its interface is designed to make network management easy for power users and professional ITs.