I have not used any such router ie. with load balancing, you may first need to find out that how this router works, ie. how it detects and then check the criteria, may be then you can find the cause of problem.
or you may...
Secondly, try to reset the router and try swaping the wan port.
or you may....
Thirdly , consult its documentation and thoroughly read its technical specification and guidelines.
or you may..
Lastly,
try updating firmware of the router.