Photo for fingering Low C. Note that all holes are closes. Also, thumb hole is completely closed.
Low C is easier on Aulos 211A-E compared to Yamaha YRT-304BII. This is the Aulos 211A-E keyless tenor recorder
Standing. If the low C fails to sound. It usually is because, either the 5th, 6th or 7th hole is not completely closed. Even the smallest leak will cause the low C to fail to sound. Another factor is the blowing. It should be very gentle and very light. It is easier if you do not articulate when blowing the Low C. Just blow out with a steady gentle breath - without articulation.
"Home Sweet Home" - the song for practising the low C.
Note that there are slurs in the song, e.g. E - F slur below. To play this slur. Blow on the E note then change finger to the F note within the same breath. Do not blow twice.
And C - B slur. Blow on the C note, then while still blowing out, change to the B note fingering.