Welcome to the forum. To help you with your problem it is for me and other community members usefull if you answer all the questions in your original post. If you answered the questions we look at the problem and hope to find a way to solve it.
1 - My ISP is b2b2c (based in Quebec, Canada)
2 - The ISP supplied a combo modem/router by Smart RG. Model number SR516ac
3 - There are no additional routers / managed swithces
4 - The network does not have VPN’s, proxy servers or high-end security
Have you tried setting up port forwarding on your router? Looks very similar to an issue I had with ARC. I manually added port forwarding to the router with the ip details and port shown on the ARC settings page in roon.
As mentioned by @Andrew_Munro, you should try to create a manual port forwarding.
First in the Roon App under Settings → Roon ARc write down the IP address and port number shown there.
Take a look at the manual on page 69 and see if you can create the port forwarding.
In the first screen click on Add
Under Interface i think you can select your Roon Core (it’s the IP address you wrote down) select that one.
Then select Custom service and enter a name ie. Roon Arc
in Trigger Port Start and End enter the port number you wrote down.
As Portocol Roon Arc only needs TCP. Try that one first. If ti’s not working, you could select *TCP/UDP
Click Apply and Save.
Check in the Roon App under settings → Roon Arc if it’s working.
Using the steps you proposed, I was able to get connected.
In case this is of potential use to others facing similar connection issues, the router had to be set to
a specific interface standard. My router proposes Eth4.1, PPP0.1 and PPP1.1. Only this last WAN Interface standard (PPP1.1) worked for connection with my Roon Core. Also, I had to use the TCP/UDP protocol.
Bottom line, the connection was activated and I am now a (happy) user of Roon Arc. Thank you for your help, much appreciated!