Part 107 UAS pilot reported their trainee realized the UAS was not displaying its registration. Pilot landed the UAS so the registration number could be applied.

Narrative

I was flying Aircraft X for an infrastructure inspection and training a new pilot to do the same. During the second flight with a different airframe than I usually fly; the trainee mentioned 'well it doesn't have the FAA sticker;' referencing the registration number. I then realized I had forgotten to apply the registration number to the aircraft. The aircraft had been registered; and therefore had a designated UAS registration number; but I failed to apply it. Upon this realization; I landed the aircraft and we grounded that aircraft until the registration number had been applied; finishing our inspections for the day with an aircraft which had an adhered registration number. Contributing to this error was the fact that I had been flying two different airframes for the prior 3 days; and with 3 days in the field I was beginning to experience some fatigue. Additionally; high temperatures and humidity were contributing to fatigue. When it came time to fly the aircraft described in this incident; I did not catch the fact that the aircraft's registration number was not adhered during pre-flight. I also realized that our pre-launch checklist did not have an item in it to check for an adhered registration.To prevent the incident from occurring again; I need to ensure that we add a checklist item for the adhered registration and be doubly careful when flying an airframe I have not been flying regularly. As always; I need to be looking out for the effects of fatigue and ensure continued vigilance when encountering them.