I use to have a network managed by a routerboard RB750Gr3 but found the internet to be very slow and when I removed it and connected the network start to the adsl router it was much faster - what i need to know is what am i doing wrong because I need to have the network managed by this device or a device similar to it
How can anyone find out if you are doing something wrong if you don’t describe what exactly you are doing?! 
Please post here your export (Open terminal and type “export hide-sensitive”) and tell us what is your download/upload speed difference.
Regards.