Force Windows to use specific network adapter to connect to NAS

I have a PC which has 2 network adapters: One on the motherboard (used to connect to the rest of the network and the internet), and a 10G adapter which is connected directly to a 10G port on a Synology RS3621xs+ NAS (which is also connected to the rest of the network via a 1G connection).

Windows sometimes uses the adapter on the motherboard, and it sometimes uses the 10G adapter - I can tell by opening task manager when doing a file transfer and looking at the network activity. Sometimes the built in adapter is doing everything, with the 10G adapter sitting idle; and at other times it is vice versa.

How can I force Windows to use the 10G adapter all the time? I did some searching and what information I found was very confusing.

Try adding a static route:
route add 192.168.4.8 mask 255.255.255.255 0.0.0.0 IF 2

Where "192.168.4.8" is the Synology's IP, "IF 2" is the interface number of the 10G NIC, as listed by "route print".

That is what I thought but I am not sure if I should use the IP address of the NAS on the network, or the IP Windows assigns to it. See the attached screenshot. The IP 169.254.202.237 is the NAS (I can find that by opening its properties in File Explorer), but its IP on the network is 172.16.10.29

I think we should back up and talk about your network layout. 169 network block is auto-assigned when you don't have DHCP on that interface (because you've cabled it directly as host-to-host).

The proper way is have your PC's and NAS 10G interfaces each statically assigned to another network, like say 172.17.0.0 so you have the same persistent addresses after every reboot. Then it should work.

Ok, that makes sense... Should I do that in Windows, as well as in Synology DSM? or just in Windows? Or is it something that has to be configured through the network?

Forgive the simple question... this is definitely the edge of what I am familiar with.

You would set a fixed IP address in Windows for the 10GB. On the Synology (logon from the 1GB interface's address), set a fixed IP for the 10GB.

NAS -> 172.17.0.2 netmask 255.255.255.0, no default gateway
PC -> 172.17.0.3 netmask 255.255.255.0, no default gateway

Makes sense... I will do that.