I have an ac1900 with ddwrt and 2 usb ports that I have 2 ssd’s connected to that can be seen by all my devices, but I don’t run linux except for occasionally playing with linux. It is more curious that your linux os does not see your routers drive - Windows should not even be involved in that, it should be linux os to router - no winos involved...
DD-WRT is DA SHIZ! Turns mere mortal rooters into Super-Rooters.
A couple months ago I was able to see the USB drive on the router with Linux. Now it won’t, and that’s what first alerted me to this problem.