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.