Two things…..
First, I believe your iPad MixPad app version and the console firmware version need to match. I would hope that a new Avantis would come with the latest firmware already installed, but double check that both devices are running the latest software/firmware versions available.
Second, we just had a band member purchase a new iPhone and he cannot get the Custom Control software to connect to the Avantis. We double checked to make sure his phone was on the local Wi-Fi network. I also install the latest version of custom control editor and saved my templates using the newest version and uploaded those to the console, but he still cannot connect even after doing that. No other musician has problems connecting to the console using Custom Control (some use Android devices and some use Apple devices). This musician also did not have problems connecting on his older iPhone.
I haven’t really had time to dive into the situation, but I’m beginning to think that it points to some issue with Apple compatibility. Perhaps there has been a very recent change that has broken compatibility.