I've had the same issue happen, running Archer C7 running OpenWRT. But I'm not 100% sure it's actually the router or whether it's really a problem with my ISP. I haven't seen the issue in a couple of months now, yet I've made no changes to my setup.

I did have considerable issues getting both wifi interfaces to work well at the same time, but that's probably more due to my own incompetence more than anything. I did finally manage to make it work, much like how you'd wiggle antennae on an old tv back in the day and suddenly you got a signal.