Well, I haven´t test it, but regarding to this document, it sounds like every radius server can be the radius. I would use Cisco as the vendor. https://documentation.meraki.com/MR/Encryption_and_Authentication/Configuring_RADIUS_Authentication_with_WPA2-Enterprise if you already have the MR´s (assuming this, as you wrote you replace the APs and not planning to), you could setup everything and within the dashboard you can check the authentification, if it works. Wireless -> AccessControll -> Radius Server
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