Playing any given tone is the process of our air pressure being able to "blow apart" the lips. If we can't play low notes, the usual situation is that the lips are pressed too tightly together. We can not just intellectually tell our facial muscles to "relax". Playing the trumpet is fine motor activity that needs many low impact repetitions to create muscle memory.
In my "circle of breath" (which is only a collection of things from others with my individual visualisation), I mention exhaling into the note. That is what is needed here. Tonguing the note adds a burst of air pressure to kick start the tone and that is NOT A GOOD THING at this stage. We should only use the tongue to "shape" the beginning of a tone (articulate). We can only start to shape when we have "tone in the first place".
Exhale into the long tones.