GS1920-24: VLAN access to internet gateway?

Hi,

I have a GS1920-24 Switch with Firmware V4.70(ABMH.6).

My plans are to setup a separate Guest-Wifi that is located on a VLAN that can only access the internet and nothing more. Until now several problems occured but I want to start with the very first.

How do I setup a default gateway (the ISP's modem/router) that is accessible for all VLANs?

My setup looks the following:
All clients are in the subnet 10.0.0.0/24, which is the default/Mgmnt VLAN 1
This includes the modem 10.0.0.254, two Unify AP AC lite and the Unify controller running on a RaspberryPI (10.0.0.241).

The GS1920 has the IP 10.0.0.240.

I added a new VLAN with the ID 19 with the tagged ports 21 and 23 (those are the ports the APs are connected to). The switch has the interface 10.0.19.240 in VLAN 19.

The new guest wifi itself works fine. I still have problems with the DHCP server, but that is another kind of problem. When I add a client with a static IP like 10.0.19.5, the wifi works fine and I can ping 10.0.19.240.

But how can I make the modem 10.0.0.254 accessible for VLAN 19 (10.0.19.0/24).

I tried to add a static route for the IP 0.0.0.0, Subnet 0.0.0.0 and target IP 10.0.0.254 which results in an error: Invalid IP address.

Do I need a separate router for my case? Or what is wrong with my plan?