Duel Wan issues

[KtBken]

Hi
I have a duel wan setup comprising of a plus net fttc connection using the bt supplied vdsl modem and a virgin media connection using there hub3 in modem only mode both 50 meg connections.
This setup has worked well for the last few years using a draytek 2960 router.
This recently died so have replaced it with a ac5300 and this is where the issues have started.
I have tried both the latest Asus firmware and am now running the latest 61 merlin firmware both have the same issue.
If i set up the router in fail over mode the duel wan works fine but if I change to load balance I then get massive packet loss ranging from 10 to 75%. This happens on both wireless and wired connections.
If i unplug ether of the wan connections the packet loss stops so assume it is some sort of internal router routing issue. has anyone else had any similar issues? As i found nothing when searching for an answer.

Thanks in advance

 

[System Error Message]

load balancing never works well on consumer routers. You may want to try configuring using files and terminal instead like you would on a linux machine.