Need help routing audio from my DAW through a guitar effect pedal using the UR-12

I recently bought a UR-12 Audio interface and I’m having trouble trying to use it to output audio from my DAW, through a guitar pedal and back into the interface.

Here’s what I got going on:

I can output audio from my DAW’s (FL Studios) mixer track into (UR 12 L - UR 12 R). I have connected the 1/4 headphone output into my guitar pedal, and my guitar pedal I plugged into input 2 (Hi - Z). The dual RCA outputs on the back of the UR-12 are connected to my monitors. Using this setup, whenever I turn the volume up on the guitar pedal I hear a loud high pitch feedback.

I’m pretty sure the issue might be that I’m essential using the same output for both the monitors and the guitar pedal. I have tried disconnecting my monitors from the dual rca output, keeping the 1/4 inch headphone output connected to the guitar pedal, and recording the audio from my daw into a separate audio track. I only managed to record the same feedback, but I am new at this, I could have been doing something wrong.

Does anyone know what could be going on here?

I’d hate to thing that the UR 12 cant handle what I’m trying to do here. But if that’s the case then can anyone tell me if I can return the UR -12 for the next more capable interface? LOL

Ideally, you would want an interface with more than just a single set of outputs.