Canon EOS REBEL T7i Instruction Manual page 308

3 Menu Function Settings
ISO Speed During Movie ShootingN
You can set the ISO speed separately for still photo shooting and movie
shooting. Set under the [z2] tab.
308
[kISO speed]

In manual exposure, you can set the ISO
speed (p.269).
[kISO Auto]

You can set the maximum limit of the
automatic ISO speed for ISO Auto to ISO
6400 or ISO 12800.
Under [54: Custom Functions(C.Fn)],
if [2: ISO expansion] is set to [1:On],
you can select [Max.:H(25600)].