Is firmware updated? Did you try change wireless password? Like change wireless security type, or no security. Would it work? That's the suggestion I can think of. It's better to contact tplink tec...

It could be compatibility issue. Did you update firmware of your modem? Tried connect the dlink access point to different ports on the modem? What would dlink say about it?

I don't think M7350 work that way.

Probblem solved. https://community.tp-link.com/en/home/forum/topic/151255?page=1&t=2018%2F12%2F20%2001%3A10%3A44%20%2B0000

There are more than one password on the back of the powerline. Did you enter the 8-digit one?

Make sure hard ware version is v1 https://www.tp-link.com/en/download/Archer-MR400_V1.html#Firmware