Hello, welcome to SG.
First of all, they are wrong - you should be able to use your ISP's gateway/modem/router and your own, as long as they are properly configured. ISPs just don't like it, and they do not support another router because it complicates troubleshooting for them. Your equipment will always the first culprit that is blamed, you can safely ignore that, but be persistent to get the issue fixed. That said your router has to be properly configured, the Asus RT-AC86U is a very capable device, with good processor and plenty of RAM.
Even though it is likely an issue is with optimum line, or equipment at your location, it may be a good idea to eliminate the possibility that it is your router - you can do that by connecting via Ethernet directly to their router. You may also want to monitor whether their router, and their ONT loses connectivity, or just the Asus router.
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