The error messages tells us that there is MultipleNAT in your configuration. That’s the problem ARC isn’t working. I’m not an expert on this subject but may this helps you (in the article there is a section of MultipleNat).
Can I check with anyone who might know if doing this would resolve the multiple NAT issue? Im keen not to change anything that Im not an expert in and could cause issues downstream?
Not an expert myself, but see screenshot below (option 2 first line). If you put the Fritzbox in Bridge mode and it’s not working, you set it back to the setting before you make the adjustment. Just make a screenshot and save it.
I would agree you have a double NAT, and your solution is correct to place the Friztboz in bridge mode , you will then need connect to your ISP with your login details via the Asus RT and then activate UPNP or add the port forward rule to the Asus RT
Bridge mode alone was not sufficient to fix the problem, not for me. So, I attempted to explain the steps I took and settings to finally get this working, at least using the same modem as you.
You have my sincerest apologies for the delayed response here, we’ve been dealing with a higher-than-usual volume following our release and we’re working as quickly as we can to get back to everyone.
I wanted to check in on this thread to see if you were still running into issues getting Arc squared away on your home network?