iPhone 6 causing wifi issue

I recently upgraded to an iPhone 8 from an iPhone 6 and a wifi issue I had stopped. But, after about 6 weeks, I reset/erased the iPhone 6, put it back on my wifi and the issue has returned.

Basically when the iPhone 6 is on my wifi, other devices will randomly be knocked off the wifi. iPads, Androids, Firesticks, Laptops even the new iPhone 8. Seems to be completely random. Turn the iPhone 6 off and the issue stops.

It's fully repeatable and had been an issue for me for months. I had no idea the iPhone 6 was the cause until now.

Any idea what specifically could be causing it?