Sorry that I have not idea.

ER6020 V2 supports IPV6. But V1 cannot upgrade to V2. Because they are different hardware version and use different hardware. You can consider to buy ER6020 V2.

For the download speed, you should consider the NAT-throughput of the router. The NAT-throughput of ER6120 1.0 doesn't reach 900Mbps. So you cannot get the 900Mbps download speed. You can consider to...

@Ajuba You can use Iperf to test the speed. If the testing speed is normal, maybe related to the application. You can connect two PC to each other directly. Then set static IP address for them and...

@vysa You can try to set static IP address for your PC. Then use the PC to ping your router. If it cannot ping successfully, it should has the problem. Maybe try to reset the switch and upgrade the...

@GuyLoucks For TP-Link L2TP/PPTP WAN VPN, it's as a dailup way, seemly used for getting IP address only. Because it doesn't support encryption, so it's hardly used for VPN. If you want to establish...