Congratulation for you. But I think it's hard to ask TP-Link to make the new firmware for version 1 router. After all version 1 is too old.

You can try to ping the webstie to check the latency. Generally enable DMZ will not get the higher speed. It's used for open the port. If you doubt that multiple NAT cause the issue, you also can...

As I know, TP-Link router doesn't support telnet. And generally we can see the router freeze, but hardly to see the router reboot automatically unless has the rebooting schedule.

Did not hear about that TP-Link has the watch-dog on their router. If confrim that the router will reboot automatically, I think you can try to get the replacement and check again.

For open all the traffic without restriction from TL-ER5120 to the Firewall, you can try DMZ as well. For redirect some services from Firewall throught one ISP, you can try policy routing....

HaoT wrote What's the performance enhancement of ER6120 over ER6020, if any? In particular, I'm interested in the ipsec throughput for both...also 3DES vs AES256 encryption for each. Data avail?...