Tankless questions are complicated because these units have many parts, and both internal and external issues can cause problems. Also problems can arise from combination of issues including maintenance, incoming water temperature, pressure, air quality, venting, water quality, water pressure, in addition to internal washers, gaskets, and parts functioning perfectly with each other over time.
Hot water only after a certain time of day indicates problem might be pressure related.
Which could be local water supply, or clog in line, or sediment on pressure sensor, failed pressure sensor, dirty in-line sediment filter, improper vent, or combination of things listed.
Rheem suggests calling service technician.
Service technicians train for years on one brand of tankless, and then see something new each day.
To understand why service technician is recommended, view number of UNanswered questions on following link showing one Rheem tankless model number.
I suggest avoiding tankless water heaters since all costs are energy.
Clogged pressure sensor after 2 years exposure to hard water. De-liming tankless will not prevent failure from hard water.
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