A router is a router... ASUS RT-AX86U is fine. There is no special provision for Synology.
The configuration is exactly the same for the 2 routers. That was one of the first things I checked. I understand how QC works, as you described. It works fine on my AC router WITHOUT needing to use port forwarding. On my AX router it doesn’t work without port forwarding. On the Synology list of compatible routers, no AX routers are listed.Have you gone through the two routers and compared their setups? Is there any new feature in the new router that require configuring?
Since the NAS doesn't use WiFi to connect to the router your issue will not be related to what wireless protocols a router supports, nor if they have no WiFi capability at all.
If you don't configure port forwarding for direct connection to the NAS from the Internet, you need QC to punch a hole outwards to Synology's QC servers which is then used to reverse proxy Internet connections through Synology (where secure connections use Synology's certificates to QC servers, get decrypted, then re-encrypted with your NAS's certificate for onward connection... because I doubt that apps and web browsers know to create a second tunnelled session through the initial disconnected connection). So this means that DSM QC acts as a client to QC server and initiates the connection and keeps it open and then the QC server can send inbound connections down this 'tunnel' to DSM. What DSM QC is doing is all normal stuff and many services use this model, but you do have to trust Synology has implemented robust security in the QC servers because connections will be decrypted in order to pass from client<->QC-server to QC-server<
What is frustrating is with the exact same settings on both routers, the AC router works WITHOUT having to set up port forwarding, while the AX router ONLY does Quick Connect if I follow Synology steps for port forwarding. Firewall and ip were EXACTLY the same. Once I did the port forwarding steps the AX router works fine. But I should not have to use two completely different methods to use Quick Connect.As I always understood, the router compatibility list is only valid when the uPnP functionality is used. E.g. When you want the DS to open and forward ports.
Using that is not safe, not recommended and a hit and miss all time.
For the rest, any router should just work Out of the box.
If your quickconnect does not work, it should be a setting like a firewall setting, fixed ip or proxy ip. Difficult to tell, a 3s network reset on the DS could clear that.
I appreciate the answers people have given, and their attempts to be helpful. When I switched to the AX router, I set it up with exactly the same configuration. ASUS routers have a good deal of settings that can be tweaked. I went page by page, and set up everything in the AX router exactly as the AC router. There are no differences in the router settings, my firewall (Norton) settings are the same, my network settings are the same. Hence my frustration.@StephenS
I have sympathy with your plight but please re-read the answers above - the issue you have is nothing to do with your Synology NAS. The guys above have been clear, honest and correct in stating this to you.
This may be a Synology focused independent forum but the members are friendly folk and will try and provide support for systems outside of the 'Syno' ecosystem. You have a router / firewall / network issue somewhere, so focus on this and explain your current network topology in order to help us to help you.
Anyway, welcome to the forum!