nand gate astable vibrator do not work at 400kHz
The CD4011 has very little drive capability, especially with a 5V supply. CD74HCU04 would be better. 

Also R1 will have to be much lower, maybe 10K or 20K, because of input and stray capacitance.

Try 100pF, 74HCU04, R2 = 11K, R3 = 20K. Probably R2 will have to be higher, maybe 15K, because of the loading from R3 but that should get you in the neighborhood. 

Unbuffered inverters are better because they are less likely to have unwanted oscillation modes.