@silverhand 

I have Deco BE65 and the newly upgraded channel selection feature is very useful. I fixed my 2.4G to channel 1. It took a while and when the neighbour routers next time restarted they all automatically "escaped" from channel 1 and now it's free for me only.

If your Deco model doesn't yet have channel selection feature I would recommend to check the following:

1) Set Deco 2.4G bandwith to 20 MHz, if it is 40MHz by default. Then it possibly can found a less noisy channel. Even 40MHz is double speed in theory, in noisy environment 20MHz is better and it has 3 possible non-overlapping channels available (1, 6, 11) when 40MHz only has 2 non-overlapping channels available.

2) Disable wifi on your ISP router. Sign in to your ISP router and disable the whole wifi radio. You can try to change the operating mode to access point only, which will also disable the wifi radios. Then it's not disturbing your Deco and only works as a wired access point.

3) Use a wifi optimizer app to see which access points overlaps your wifi channels. The only non-overlapping channels are 1, 6 and 11. If possible, you could try to agree with your nearest neighbours if they would select one of these channels and then hope that Deco will automatically select the empty channel left. 

You can also try to restart Deco until it finds a better channel and use the wifi optimizer in Deco app that should change to a better channel, if it's not optimal.

  @silverhand 

As Deco S4 doesn't have the manual channel selection feature I guess the best possibility to solve the issue is to shut down the wifi radio of your old ISP router. If I understand correctly you are routing your ISP network wirelessly using the Decos and the ISP modem is only running with a useless 2.4 GHz radio signal. I understand you cannot run two wireless networks at the same time. In my opinion, the best way would be to shut down the wifi radio in your old ISP router and use it only as a wired access point and let the Deco handle the wireless network.

Another thing is to minimize the number of devices that connect to your 2.4G wifi. Use 5Ghz instead for all devices that can use it. Then the devices in 2.4G wifi will get as much bandwith as possible. How many devices you have that connect to your 2.4G wifi?