AC56u behind modem/Router - no working dhcp - wifi disconnects - pls help

hi
iam getting crazy...maybe u can help me.
i think there are many good guys here who can solve my problem
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what ive got:

-asus ac56u router
-O2 router (just a stupid modem/router from my internetprovider)
-advanced tomato 138
-connection between modem -> ac56u = lancable to lanport 1 of ac56u


the configuration i want:

dsl->modem/router (wifi deaktivated.dhcp deactivated.just getting the dsl.sending it forward to the ac56u)->ac56u->clients

so i want the...

AC56u behind modem/Router - no working dhcp - wifi disconnects - pls help