Hi,
I am using 2 routers. Router one (where the NAS is attached) has the IP 10.0.0.1 and Subnet 255.255.255.0. I run Home Assisstant on the NAS in a virtual machine.
Router 2, where all the IoT devices are running has 192.168.68.1 and Subnet 255.255.255.0
I am not able to access to Subnet 2 and already tried with the routing table, either on the NAS or on the router, to get access to this network. But without any success.
In my routing table I have
Target 192.168.68.0
Subnet: 255.255.255.0
Gateway: 10.0.0.2
Hope you can help.
Router1: 10.0.0.1
Home Assisstand: 10.0.0.5
IP to second router: 10.0.0.2
Router2: 192.168.68.1
Devices on 2nd network: 192.168.68.x
I will try gateway gateway 10.0.0.1, but from my understanding, I need to name the IP, where the traffic should go out.
I am also wondering if and where I have to tell Home Assisstant scan also this IP range for devices?
Why are you using two routers? You will only face issues with two active routers with dhcp and so on.
If you want two different networks you should use ONE router, define f.ex VLAN10 192.168.10.0/24 and VLAN20 192.168.20.0/24 there and set up the traffic and isolation between the vlans.
Is that second router a NAT device that won’t pass traffic through to the isolated IoT 192.168.68.0/24 subnet???
I gave up with my ISP. 
The router had massiv issues to keep Wifi stable. Switched 3 times the router without enhancements and decided to buy a good one. This stupid router doesn’t have Bridge Mode, which would make my life easier. So two router with two DHCP is my life…
This is my assumption. But in my world Static routing should fix this. But seems not and I am wondering if I did something wrong in the setup.
You should definitely set this up differently, this is a recipe for trouble. If you have no way of setting your ISP router in bridge mode at least connect everything to your own router, do not keep some devices on one router and others on the other router. Have you looked into bypassing the isp router? Where I am located this is perfectly doable (connecting the fiber straight into your own router and setting vlans for example).
If you keep all your devices on a single network you should have no issues on the internal communication. Double NAT comes into play when you want to expose services outside your LAN. If the idea is to keep isolation and separating due to security you should set that up on your own router and keep the ISP one out of the picture.