Not Able to reach Gateway through a Nexus

Hey everyone,

somehow my brain stucks. It seems so easy but... idk...

Scenario:

I got a MPLS Router which is connected to our Nexus Switch. The Nexus is the Gateway for the MPLS Router. Our Firewall also is connected to our Nexus Switch.

Now i want to set the Firewall as Gateway for the MPLS Router and get rid of the Gateway Config on the Nexus. The whole Scenario is nothing less than a Router on a Stick Configuration, isn't it?
Somehow i cant reach the Gateway on the Firewall through the Nexus. Sure i could plug the MPLS Router directly to the Firewall, but in the Future we want to do Segmentation via Trunk with our Firewall.
so i connected my Router to the nexus via trunkport and connected the firewall to the nexus via access vlan where the Router have an interface in. so the whole Thing is a L2 Connection... Before you ask, neither trunk or access port is working. I'm confused now...
IDK but somewhere i'm stuck in this simple config... Help! :(