Which mode did the 5210 work? Can you Ping the 7210 from 5210? Have you setup distance parameter in the 5210?

stupidbird wrote

Which mode did the 5210 work?

It's on #1

wa5210g set as client

You can log in into all three boxes, i.e. ping works all over.
It seems to me 5210 started one day to screw up ip packets and information contained therein (wan, dhcp requests).
Reset to factory defaults = no help.
The link speed is roughly 20 Mbps btw 5210 and DSL router.

edit: Does it make any sense to do long reset (30s holding reset button, unplug electricity, still hold 30 s, plug in electricity, release after 30 s)?

I've never heard about what you mentioned of 'long reset'. I don't think this operation makes sense.
Is your environment with severe interference? Use a Wi-Fi analysis tool such as inssider to detect.
And remember to setup distance parameter. This is important.

@stupidbird, 30/30/30 reset it's for Broadcom routers
1. UNLESS YOU HAVE A LINKSYS EA ROUTER, DO A HARD RESET *BEFORE AND AFTER* YOU CHANGE DD-WRT FIRMWARE VERSIONS. This does not mean hitting the reset button and saying you are done. This means doing the 30-30-30 reset. To do a 30-30-30 reset you must push the reset button with your router powered on. Hold it for 30 seconds with the router powered on. STILL holding it, pull the power cord for 30 seconds. Still holding it, plug the power back into your router and continue to hold the reset button for 30 more seconds. You will have held the button for a full 90 seconds without releasing it.
You

this has something to do with NAT and DHCP the distance parameter is 1 as it was befor4e (real distance 800m line of sight)

sbf wrote

this has something to do with NAT and DHCP the distance parameter is 1 as it was befor4e (real distance 800m line of sight)

have you tried dif cable, reload firmware, try it where you have the AP to discard issues, if you point to only one issue, you might never find the cause.