Yes remove any load balancing you have or reduce to the minimum level. You could also set the load balance policy up for the internal bes server.
On the draytek try Advanced tab > NAt > address mapping that should do the trick. - se;ect protocol All then pick the public IP
Or even better !!!
Advanced - nat - DMZ host. - select the wan then enter the private ip, it will let you even select an IP alial if your ISP was nice enough to give more than one IP.
Hope this makes sense.