RouterOS Double NAT help (noob)

Hello there, im new to RouterOS, and even if i was able to do almost everything i wanted, im not being able to setup a double nat (yes i know that s*cks, but i still want to know how to do it).
Basically, i have a non RouterOS device that connects to my ISP via DSL, this router does NAT and everything i need. Then i have the wan port of a routerboard hex lite connected to one port of that first router, with a static IP and a few clients that connect to it via pppoe, however it is still the first router (the tplink one) doing the nat, and i can not figure out how to make the RouterBoard do a double nat against the private (10.0.0.2) wan interface.
Can anyone guide me into the right direction ?
Right now, this is how my nat rule looks like:

 /ip firewall nat print 
Flags: X - disabled, I - invalid, D - dynamic 
 0    chain=srcnat action=masquerade out-interface=WAN log=no log-prefix=""

And this is my routing table:

/ip route print 
Flags: X - disabled, A - active, D - dynamic, C - connect, S - static, r - rip, b - bgp, o - ospf, m - mme, B - blackhole, U - unreachable, P - prohibit 
 #      DST-ADDRESS        PREF-SRC        GATEWAY            DISTANCE
 0 A S  0.0.0.0/0                          10.0.0.1                  1
 1 ADC  10.0.0.0/24        10.0.0.2        WAN                       0
 2 ADC  10.0.100.251/32    10.0.0.2        <pppoe-test>              0
 3 ADC  10.0.100.253/32    10.0.40.2       <pppoe-norita>            0
 4 ADC  10.0.100.254/32    10.0.40.2       <pppoe-gato>              0
 5 ADC  10.0.100.255/32    10.0.40.2       <pppoe-fer>               0

I am not sure what i might be doing wrong, but so far it is still the first router the one doing the nat for the connections to the mikrotik one, any hint ?

Thanks!

Post the rest of your configuration - what you have looks ok to me