How is the 810L configured and connected to the 645?
Did you follow this? Turning a router into an AP.
How is the printer connected and configured on the network? Wired or wireless?
Is the 645 and 810L using different channels on 2.4ghz?
The printer is connected wirelessly, the 810L is setup as in the page you linked, and is working fine, I just did the firmware update and am rechecking it's settings, so far so good.
Edit: I just rechecked my laptop settings, it was set to work as if the printer was connected by usb. Fixed that and everything is working.
You can delete the thread if you want as it turns out that it is not needed.