In case anyone needs it here the link:

https://odootips.com/ip-estática-en-posbox-8843a763216

Here an extract:

sudo ssh into raspberry (you can look the ip if you attach a monitor to it), then execute this:

sudo mount -o rw,remount /
sudo mount -o remount,rw /root_bypass_ramdisks
sudo nano /root_bypass_ramdisks/etc/dhcpcd.conf
ADD to the file in the last lines:

interface eth0 <--- this is the name of your card, and here the static ip is .150
static ip_address=192.168.1.150/24
static routes=192.168.1.1
static domain_name_servers=192.168.1.150

after you save the changes, reboot and done.

If your RPi get the IP from the DHCP, on your router/DHCP may have a feature to fix the IP to MAC address.

Thanks guys

That was simple enough!