I asked another user as well but is it not possible to load mods in single-player via GeForce Now? I am not familiar how it works.
On another note, the mod loader seems to be working properly for me. It shows me the mod mismatch, it loads the mods, restarts my client and connects to the server. It is not an universal issue but rather something specific to those using GeForce Now.
Some additional info by Testerle: