I want the laptop with IP address 192.168.75.13 to be able to access the file server 192.168.10.254, but still keep the internet connection of the laptop through the SR520. I'm not sure what modifications I need to make to the SR520 to link the two subnetworks (192.168.75.x and 192.168.10.x).
From your network diagram, I do not see a L3 device between 192.168.75.x
subnet and 192.168.10.x subnet. The easiest solution would be for you to
configure a secondary address on your Laptop so that it will be part of both
subnets. You can still keep the 75.1 address as your default gateway.
Hope this helps.