1. Where did you see speed 450Mbps? Provide a screenshot? What's the wireless speed of wireless card on your clients? 2. Did you check DHCP Clients list on D9? Only devices connect to it directly by...

That's old. When did you purchase it? As internet works fine and we just need to log in its web interface sometimes, you may leave it. When you want to manage it, just restart it.

Firmware of your ER6020 is up to date? On previous router, did you configure anything to make the telephone server work? You'd better confirm with the provider of telephone server. Provide the...

Any update on this issue? You cannot find it under network adapters or other devices, right? It should be your PC's problem. Is it possible for you to verify it on a different PC?

If you simply reset the router, it will work in bridge mode. Did you do troubleshooting with your router's technical support? WAN IP is obtained on the router? Is it a DNS issue?

NAT is enabled in default settings. Are you trying to disable NAT? Unfortunately, it has no such an option.