If the subnet mask is 255.255.255.0 for both, then 192.168.0.0 and 192.168.1.0 are different networks.
If the freeview box has a fixed (i.e. manually / statically assigned, not DHCP) IP address in the 192.168.1.X range, and you connect it to the secondary, it won't work because it's got the wrong IP network address for that network (192.168.0 is not the same as 192.168.1).
You would need to either set it to pick up a 192.168.0.X address via DHCP from the secondary router (ensure that the default gateway address - 192.168.0.1 - is also assigned correctly by DHCP) or else manually set its IP address for the secondary network as 192.168.0.X (choose an X between 2 and 254 not already in use!), default gateway = 192.168.0.1 and subnet mask = 255.255.255.0 (assumed).
You will also need to supply a DNS IP address - this could be 192.168.1.1 if that's what other devices on your network use - it doesn't have to be on your network, or even from your ISP, it can be internet based, e.g. OpenDNS. There's an interesting selection here:
https://www.techradar.com/uk/news/best-dns-server. jimjim1's example has Quad 9 as DNS (9.9.9.9), which is another of the free DNS services mentioned.