@DnAHousehold 

Okay since you are using a Gateway the first question for us would be, what mode are you using the Deco in?  Is it in a router or AP mode? The next question would be, what node are you connecting to and how far from the node are you?

  @Carl I have it set as a WiFi router and the gateway is set so it is only acting as a modem. The main deco is right beside the gateway and plugged into port 1 on deco which goes to lan1 on gateway. It's been configured and working great for past 8 months.

  @DnAHousehold 

Okay good, so we know it's not a Double NAT issue then.  My next question would be: Have you or are you able to test the speed via Ethernet directly to the main Deco?  

  @Carl I had not checked that but I did just now. Speed test connected via Ethernet to main deco was 258.21 download and 50.66 upload.

  @DnAHousehold 

Okay and your ISP speed is 300 down 30 up right?  Additionally when you were testing the speed wirelessly and got 90 Mbps was that on 2.4GHz?

  @Carl the WiFi speeds I was getting on average were in the low 200's for Upload and downloads on average at 20. What the ISP says is typical download speeds between 33-182 Mbps. So I am at this point just trying to rule out or in if the firmware update may have had a negative effect. This is a 5G home internet connection. I have been sent a new gateway that I haven't been able to setup yet. Just trying to cover all bases as my speed's drastically dropped after that firmware update.