Hello.
I have a router from my provider that I can't change because of hardware limitations.
That being said the software side of it is well lets say not so good. That's why I want
to use my OpenWrt TL-WR841ND V.9 router as the DHCP and DNS server behind the Main router
which only provides the internet connection.
When I connect the OpenWrt router to one of the lan ports of the Main router and my computer with the OpenWrt router its DHCP server is ignored and the Main DHCP server is used.
Maybe you guys can help

I presume you connect the WAN-port of the TL-WR841ND to a free LAN-port on the main router, and your computer to one of the LAN ports of the TL-WR841ND.

The symptoms you describe indicate that the WAN and LAN ports of the TL-WR841ND are linked together. Thus, the DHCP requests coming from your computer are served by either DHCP server, either the one in the TL-WR841ND or the one in the main router, whoever manages to do it first.

Start by checking if TL-WR841ND has an in-built switch device (swconfig), then proceed by determining if the switch device is configured correctly. The switch should put the physical LAN ports 1-4 into a separate VLAN, and the WAN port to another VLAN, so traffic doesn't get relayed directly, and instead passes through the OpenWRT router's kernel.

That worked for me. Thank you very much