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So I've been wondering from the beginning whether there might have been a change implemented in this latest OS release that may have broken the SSH compatibility with PuTTY.
THIS IS AN EARLY ACCESS PRODUCT!! But go ahead and lump everyone together.
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My understanding is that the device authentication credentials on the page you cited are for SSH connection to access points and other network devices, whereas the credentials at the page I depicted here in this post are for SSH connection to the UDM Pro console itself.
For me both username IP-address or IP-address alone works AceOfSpades, I would be interested to know whether you are able to connect via PuTTY or plink to your UDM Pro OS console vs.