I had the exact same problem, but with a Pixlite 4 MKII, and I reported it to Garagecube support. Connecting the miniMad directly to the Pixlite simply does not work, regardless of the DNS settings I defined for the MiniMad or the settings on the Pixlite (I tried dozens of combos of all settings).
My solution was to place a network switch between the miniMad and the Pixlite, and things suddenly started working. Based on Advatek's Pixlite manual, I got a switch with IGMP support, specifically this one: https://www.amazon.com/gp/product/B07BN ... UTF8&psc=1
Connecting the miniMad and Pixlite to the same network switch solved my issue. But it would be nice if I didn't have to do that..
P.S. The MiniMad documentation should probably make a note about Advatek controllers