I have had this issue on the last 3 srp521's I deployed, all were pulled from service and replaced with a different model. Firmware is 1.1.24 and I also tried on with 1.1.27
With only power and 1 lan connection I start a ping to the router and a ping through it to the DG of the wan ip. When I plug in the wan cable I get a single reply from the wan ip DG and then my lan connection drops. I have to power cycle the router to get it to come back. I have a static set on the WAN and am using 10.10.10.x on the lan side. The link light on the router stays on for both lan and wan.
Any idea on what to look at?