Nighthawk mesh slow speed

If I am reading this correctly, you are connecting the Nighthawk router via ethernet to your ISP router. I would suggest shutting down the wireless services on you ISP router when doing this to eleminate any interference in the wireless signals. Before connecting the Nighthawk, I would take a laptop and connect it to the ISP ethernet port and do a hardwired speed test. Let's say you have 1 Gig comming down to that port. Now, connect the Nighthawk router and do a wireless speed test with your phone. If you have a Wi-Fi 6 phone, you should get between 500 and 800 Mbps. Yes, you lose at least 20% off the bat with the best phone. If you have a wifi 5 phone you will get around 300 Mbps, or maybe up to 500 Mbps. If you have satellite sysyems, that reduces your speed even further. I use the Nighthawk as the primary unit being a simple cable modem. On the main router, I get 800 Mbps with my Wi-Fi 6 phone. But I only get around 300 - 500 Mbps behind the meshed satellite units.