  Quasi-periodic oscillation (QPO) signals are discovered in some fast radio
bursts (FRBs) such as FRB 20191221A, as well as in the X-ray burst associated
with the galactic FRB from SGR 1935+2154. We revisit the intermediate-field FRB
model where the radio waves are generated as fast-magnetosonic waves through
magnetic reconnection near the light cylinder. The current sheet in the
magnetar wind is compressed by a low frequency pulse emitted from the inner
magnetosphere to trigger magnetic reconnection. By incorporating the wave
dynamics of the magnetosphere, we demonstrate how the FRB frequency, the single
pulse width, and luminosity are determined by the period, magnetic field, QPO
frequency and quake energetics of the magnetar. We find that this model can
naturally and self-consistently interpret the X-ray/radio event from SGR
1935+2154 and the QPO in FRB 20191221A. It can also explain the observed wide
energy range of repeating FRBs in a narrow bandwidth.
