HAproxy 503 issue using same configs on multi region

We have HAproxy v2.0 in AWS east region and using it for last few years, we are working on deploying HAProxy v2.9 on west region and trying to setup weighted routing from R53, everything is working until the final layer, HAproxy LB connectivity is completed but for some reasons seeing 503 NOSRV available in logs, we are using same config what we had in east region except the backend config of having west region and corresponding west urls, can anyone point us in right direction on those 503;s