How to change Gateway IP Adress

Hello community,
I cannot change the gateway IP Adress the Raspberry uses.
My raspberry with Nextcloud on it has IP 192.168.1.212. The gateway (Router) is 192.168.1.1 but the nextcloudpi after boot has always 192.168.0.1. as gateway. It’s also shown in the web interface
This is the output of ip route list after boot:

default via 192.168.0.1 dev eth0 src 192.168.1.212 metric 202
192.168.0.1 dev eth0 scope link src 192.168.1.212 metric 202
192.168.1.0/24 dev eth0 proto dhcp scope link src 192.168.1.212 metric 202

Then webinterface is very slow and Internet is not reachable. When I change the gateway via

sudo ip route delete default
sudo ip route delete 192.168.0.1
sudo ip route add default via 192.168.1.1 dev eth0 src 192.168.1.212 metric 202

its running very much better.
Then the output is

default via 192.168.1.1 dev eth0
192.168.1.0/24 dev eth0 proto dhcp scope link src 192.168.1.212 metric 202

I already tried to change the /etc/network/interfaces and also created a /etc/network/interfaces.d/eth0 but nothing is working. Please can someone tell me where nextcloudpi has the configuration for the used gateway?

There is the documentation here:

It looks like by default it uses DHCP from your router, but you could use a static ip as well. If it is DHCP, it is your DHCP that provides the ip address and the gateway.
Aren’t there option in the nc-* commands? It is possible that they update the system files behind that, and if you change them as well, that can get a bit messy.