I am not 100% sure, I do not know, and I have returned this product because it was not as good as the hype and spec that it is made out to be. I recommend Synology RT2600. it is all round a better cheaper product and to answer your question, with the syno 2600 Yes you can mesh.

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Unifi FlexHD works well. Also I am using Netgear APs with my UDM. UDM has trouble assigning permanent IP addresses to the clients connected to the Netgear APs.