Install OpenWRT on Fritzbox 7530

Hi, I am trying to install OpenWRT on my fritzbox 7530. I am following the guide but i can't manage to flash the uboot into the device.
My router is on IP which is my default gateway and my computer is the only device connected to the network directly with an Ethernet cable on port 4. The ip address of the device that I am using is
I have downloaded the uboot file from

and the from github. I am following the procedure from Linux Manjaro.

When i run the script
./ --offset 0x85000000 uboot-fritz7530.bin
i get a credential error in the first place but i've managed to fix by replacing my router credential into the python script in line
ftp = FTP(args.ip, "user", "pass")

Right now i am getting another error which is

ftplib.error_perm: 500 'SETENV memsize 0x07f80000': command not understood

Does someone has a clue about what is going on and how to potentially fix this error?
If you have any ideas please let me know. Thank you in advance.