Some help with a draytek configure please.

I'm running a draytek 2925 router (with wifi) and I have two ADSL lines for capacity, defined as load balanced set, therefore I have 2 x 24mb ADSL fibre with combined download of 45mb and 9Mb upload.
Firmware : 3.8.8.2
WAN 1 have x.y.z.85 x.y.z.86 x.y.z.87 defined on it.
Wan 2 have a.b.c.162
Load balancing is set to : Auto Weight : Session Based.

I have traffic flowing over both wan ports and all the machines in house use this capacity.

I also have two Plex Media Servers which has Ports Forwarded via NAT / Port Redirection, to connect with the plex interent services. I'm getting issues with Plex Media Servers disconnecting as they 'jump around on the WAN interfaces.
Nat Port Redirection Rules:
1. x.y.z.187 : 32400 -> e.f.g.20 : 32400
2. a.b.c.162 : 32500 -> e.f.g.22 : 32400
(I know I am using a different 'public port' but I've discovered this 'sort of works with plex' rather than me 'pinning' the correct outbound traffic.)

How do I 'Pin' so the outbound traffic is alway 'send' from the 2925 as correct IP address rather than using 'random' load balance IP Address.
Eg :
Outbound from
e.f.g.20:32400 -> x.y.z.187 : 32400
e.f.g.22:34200 -> a.b.c.162 : 32400

If someone could suggest a config please that would be great.

Regards