As @alexcherry said, Splynx is just an radius Billing system.
PCC / load balancing is done by the Mikrotik Routerboard. so if load is not distributing among all wan links, its the mikrotik that you should troubleshoot.
In PCC, there is a option called classifier, it decides what type of load balancing you want to achieve. there are 8 types.
If users request are directly hitting Mikrotik configured with PCC , then you will get good load balancing. Try both addresses and ports approach as the classifier. While this will randomize things the most and in theory give you the most fair allocation of bandwidth, BUT there is also a good chance that it will break certain things like banking web sites and some forums. This is because often times a HTTP requests will generate several connections, so there is a chance that some requests may go out a different route than the initial one, and that will break secure web sites. For that reason I usually stick with src-address for PCC load balancing. in src-address approach user will auto distribute to wan links and will stick to that wan it was attached initially for that connected session. means a single suer wont be able to use all wan links.
IF with both-address-and-ports approach you are still unable to distribute load among all wan links , then it must be MANGLE section issue, check all mangle rules that thay are counting or not.