Having issues with Ubiquiti devices and DHPC leases from an upstream 2862N running stateful DHCPv6 server

Ubiquiti device as client does not get DHCP address
other clients PC, MAC do.

Read this article
https://help.ubnt.com/hc/en-us/articles/115005868927-UniFi-USG-Addressing-How-to-Implement-IPv6-with-DHCPv6-and-Prefix-Delegation

I don't know about the 2862N, but I have a 2862 and have tried getting IPv6 prefix delegation/sub-delegation to work. In my situation my ISP delegates a /48 prefix to the WAN side of the 2862 (this works fine), and I have tried to get the 2862 to (sub-)delegate multiple /64 prefixes (effectively subnets of the /48 prefix) to a linux box on the LAN side which has several network interfaces each of which requires a delegated prefix.

I have succeeded in getting one /64 prefix (sub-)delegated to one of the network interfaces but the 2862 ignores the other requests. I have raised a support case and after providing various information managed to get a comment from development that there were some aspects that hadn't been fully implemented yet.

I also tried getting a /56 prefix (sub-)delegated instead so I could split this up into /64s myself but the 2862 wouldn't delegate anything then.

My support request #DQ905174 is now in a queue as an enhancement request waiting for resources. So if this functionality would be useful to you I suggest you submit a support request to add your vote to raise the priority of this work.

I tore my hair out with this one but finally got it.

I got my 2860 to have a static /59 (carved from ISP given /56) on LAN2, and delegated /62 of that for downstream.

This worked fine for me. The downstream device (a watchguard) needed at least a /62. /64 wouldn’t work.

Spent many hours on this, but worked for me.

I did eventually manage to get the 2862 to delegate one /60 address which I could then split into /64s. Like you it took a lot of effort, not helped by the lack of documentation and also the fact that you can configure some settings that it will silently ignore - error messages would help.