I have the router now. Here is some back experience (MR8300 v1.1)
start the router without WAN access. go to 192.168.1.1. Click on background to bypass the loading of controler for your smartphone.
Go to 192.168.1.1/fwupdate.html and upload a factory image.
First I try to upload a custom build image (with mesh), and the wifi0 (qca 9886) doesn't seem to work. There are documented issues about this (EA8300 and MR8300). I Tried to change channel, power, country ,driver (CT or not). I went back to wpad-basic-wolfsll (no mesh), nothing seem to work. ipq4019 internal wifi works nevertheless.
So I switched to the 2nd firmware which still has the stock firmware, and setup the LAN and wifi with it.I allowed WAN access. I flashed the factory image provided by openwrt (no custom). I remove wpad-basic-wolfsll to wpad-mesh-wolfssl. Everything works now, including the wifi0.
There is certainly something that had change between the two tries, but I can't tell.