The errors message is clearly telling you one of the devices are on a different subnet there is way to reach that device if they are on different subnet
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I just set up raspberry pi-hole as well on my WNDR3700 running dd-wrt I'm running debian buster I run in to some issues finally got it up and running
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I was freaking out one day as well and I was like I need to go get a beer I was gone for at least 35 minutes when I came back a message pops up firmware uploaded successfully please reboot router manually everything went well RT-N66U
U,R, B
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On this specific router you have to be patiently because it takes at least an hour to upload firmware let it do its things you will be fine
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Download the firmware you would like to install then put the router on safe mode https://www.asus.com/support/FAQ/1000814/#
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Dude in order to update firmware your router needs to be factory reset and be on the 192.168.1.1 IP address were in the world you got that IP address
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