RAX48-100NAS does not recognize internet cable. (AX5200)

I have a Netgear N750 wireless dual band router Model WNDR4300 and wanted to upgrade for better security and speed reasons. So, I purchased a new Nighthawk AX 5200 AX6 series router. Model: RAX48-100NAS. I downloaded the Nighthawk App to my phone and set up the new router. When I got to the section where the app checked for the internet connection it said in red letters that there was no internet connection to check the cable. I checked the cable and all looked good, but I switched it out for the one that came with the new router, and tryed again. Same problem. I tried plugging the internet cable back into my old router router and rebooted, internet came right up and worked. At this point I called Netgear support, strong Indian accent, but was very helpfull, we routed cables from the gear ports to the internet ethernet port, we manually set up the router, we tried different cables, always getting the no internet connection report. Finally, the tech assistant said he was sorry but he thought my internet port on the router was faulty and I needed to return it to Best Buy where I bought it for a replacement, I did this and now I have the same problem with the replacement router (same model). The internet cable works fine on my old router but not on my new. The directions say to recycle your cable modem but I can't do that because I have light cable to my home and there is no modem in my house, it is outside in a locked box on my house. I called my ISP and asked if there was something they have to do to get it to work and they said no, I should be able to change out routers at any time. So I thought I would give one last effort here to get this router to work before I return it for my money back. Any suggestions?