How close to you does a 5e Familiar have to be for you to be able to dismiss it into its pocket dimension? Within 30', to match the area you can resummon it into? Within voice range? Within 100', which is telepathic communication range? Within 120' if you use the Message cantrip to make the command? Within any distance on the same plane, if you have Voice of the Chain Master? Within any distance on the same plane, regardless? Within any distance on any plane, if you cast Sending to make the command? Anywhere in the multiverse? The Find Familiar spell does not say.
You seem to be operating under the assumption that the "dismissal" of the familiar is a command that it must be given. But the spell does not require the familiar to be communicated with to be dismissed. According to the PHB, p. 239:
As an action, you can temporarily dismiss your familiar. It disappears into a pocket dimension where it awaits your summons. Alternatively, you can dismiss it forever.
This is the start of its own paragraph. There is no qualification given for distance, so it can be done at any distance. Contrast this with the qualified other abilities of the familiar, such as needing to be within 100' to cast a spell through it, or within 100' to communicate with it telepathically.
This has been confirmed in Sage Advice.
Jeremy Crawford: Find familiar puts a range limit on the telepathy it provides. It intentionally puts no range limit on dismissing the familiar.
"Tobbun": So if you don't know where your familiar is, you can just dismiss it and call it forth again near you?
Jeremy Crawford: Yes.