Gateway doesn't work in IP range 172.16.40.0/24

[Gunnar]

Hi,
i am using a Banana Pi with 3.4.90 and the gateway in my config doesn't work:

[Match]
Name=eth0

[Network]
Address=172.16.40.102/24
Gateway=172.16.40.1
DNS=172.16.40.20

with this config it's works fine:
[Match]
Name=eth0

[Network]
Address=192.168.40.102/24
Gateway=192.168.40.1
DNS=172.16.40.20

When i add the Gateway in cli "ip route add default via 172.16.40.1" it works fine, but after reboot its lost of course.
Of course i have 2 IP ranges i am working with und of course the gateway works for other systems.
Same Problem by the way when i give the ip address and gateway over dhcp to the system

Greetings Gunnar