Hi. What i noticed in your heat transfer node is that there is no temperature node. i think that is the one causing the problem.
I suppose your fluid is your heat source to create the thermal difference in your geometry. But since you don't know what will be temperature of your solid part, so I would suggest to use the conjugate heat transfer instead and not the heat transfer and non isothermal flow.
Two temperatures for the hot and cold might work in your case.
Maybe the pictures that I will be attaching might help.
I suppose your fluid is your heat source to create the thermal difference in your geometry. But since you don't know what will be temperature of your solid part, so I would suggest to use the conjugate heat transfer instead and not the heat transfer and non isothermal flow.
Two temperatures for the hot and cold might work in your case.
Maybe the pictures that I will be attaching might help.