@Glennyrd 

A couple of things you can try if you have not already done so... Factory reset the router and reconfigure.  While I have not seen a problem like this on newer routers, in the past, I have seen firmware updates that did not "retain" configurations properly causing weird behavior after the update.  Factory resetting helped in these cases.  Also, I wonder if the router is choosing the best channel for the 2.4GHz band.  Try to manually assign it to something other than channel 6 as it is in the center of radios overall BW and therefore a common default channel for routers.  If you know how to use a wifi analyzer app on your computer or android phone, that can help you find the best channel with the least amount of neighboring interference.

I hope this helps.