Had this router a few months and the 2.4ghz WiFi has never been great, between 20-30mbps but it's usable.  Only recently it has been slowing down to 1.2mbps or lower sometimes.  Signal strength on my 2.4ghz phone is always excellent but link speed is usually between 54 - 72mbps.  We don't have many WiFi networks here so it's not congestion. 

My 5Ghz network is fine and gets between 250-500mbps (my max) usually.  I am trying some different settings now, seems to be working OK at the minute.  I'm using b/g/n mixed mode, channel width 20mhz, wpa2-psk (aes) transmit power middle, channel 1.  Was previously using b/g/n/ax mixed mode, channel width auto, wpa3-pesonal+wpa2-psk(aes) transmit power high, channel 11.