Hate to be the bearer of bad news: although your dac has an I2S input, for sure your computer doesn’t have an I2S port to output that signal. Without an additional device, it wouldn’t work.
Next, if you wanted to playback DSD from your computer, you would need files in the DSF format. These can also be transferred to your dac over USB.
As for the sampling rate, did you check the dac settings? I pressume, these should be set to auto, or similar.
From what I see, the Stellar is a newer version than the Gain Cell and they’re not the same. I can’t find any firmware in the Gain Cell dac. So, if you’re really interested, find your current firmware version and call on PSA to find if you’re having the latest version.
I dont know the SMSL device. I do use a Matrix Audio X-SPDIF 2 with my Stellar Gain Cell DAC and it works great fed by USB from my streamer. They can be found used easily as the Matrix has a version 3 out now.
If you are using a Mac - check audio midi settings in utilities and change the sample rate there.
Before PSA PST, I used a similar converter (cheap) from a Oppo 205 player to DS MK1. The sound sucked to put it simply. You will hear SACDs sounding not better and probably worse than CDs.