Tankless going cold in the shower

Hello, I'm wondering if you all can help me.

I'm in a new house 2 years old with a tankless Rinnai v75 tankless water heater. We just had a Kinetico 2030s water softener installed and now we are having trouble with the water heater. When running the shower, I have hot water for about 3-4 minutes before it goes ice cold. If I press the button to recirculate, I get about 3-4 minutes of hot water again, then it goes ice cold.

Any ideas here? I'm not sure if this is related to the softener or if it's a coincidence. What should I investigate for this? Thank you for your help.

 

Depending on how cool you like your shower, if you have a low flow head, you may be on the edge of the minimum flow rate, and the tankless no longer thinks you’re using hot water. The recirculating pump increases the flow, but may cycle, and when it shuts off, you return to the same situation. Just a thought...