Firstly, it's unfortunate that you're facing difficulties with the support from Minisforum, I have recently encountered their support also when I asked if 96Gb DDR5 was supported for on my NPB6 and they said no, but I bought it anyway and it works just fine, uptime over 3 months now.
When it comes to mini PCs, especially the more affordable and expandable ones, it's crucial to set realistic expectations for after-sales support. Companies known for their robust customer service infrastructure may be a safer bet for you if you prioritize technical assistance (Cisco, HP, Dell).
Now, regarding the specific issue you're facing with the non-working E810-XXVDA2, it's important to consider the compatibility of the hardware. Mini PCs often have limitations in terms of validated components, especially components intended for datacenters. In this instance, it's worth noting AMD probably never validated this CPU with that card, many bugs and gremlins could be there and nobody ever saw, cant really blame Minisforum for that, its way above their head.
My suggestion would be to check whether the 25GbE network card you've chosen has been validated by Intel for use with that AMD CPU or vice versa. Intel or Amd, being a major player in the hardware industry, may have insights or solutions to make your card work seamlessly with their CPU.
While it's frustrating to face issues with a new purchase, exploring multiple channels for support, including the original component manufacturer like Intel & Amd, might lead to a more satisfactory resolution.
Good luck with finding a resolution and dont forget to share with us
PS .. with regards to ordering from them directly, here in Europe we have 2 weeks to send back for a refund no questions asked.