From memory it’s in the settings for the shares themselves - one of them is whether the recycle bin is turned on for it or not. Once enabled you’ll see it as an additional share named something like Recycle bin - Volume_1 (if you are using JBOD mode you may also see Recycle bin - Volume_2 for the second disk).
If it’s not enabled for the share that contained the files you deleted then they’re gone for good as far as the device itself is concerned, although you may be able to recover them using 3rd party software (although I would be amazed if such software can do that over a network).
