Can I see through my Familiar's senses/communicate with my Familiar if they are on the other side of an Interdimensional Portal?

Question

This question has been created to help adjudicate the question of whether a Familiar may communicate with their master inside of a Bag of Holding.

Suppose that 5' away from a Wizard is a portal to the Feywild. On the other side of the portal, their familiar is 5' away from the corresponding portal. The distance measured between them, using the portal, is only 10'. Does this satisfy the requirements of Find Familiar in its stipulation that its features may be used while the familiar is within 100' of the caster, or does the fact that they're on separate planes of reality mean that their distance is greater than the "walking distance" through the portal?

While your familiar is within 100 feet of you, you can communicate with it telepathically. Additionally, as an action, you can see through your familiar's eyes and hear what it hears until the start of your next turn, gaining the benefits of any special senses that the familiar has. During this time, you are deaf and blind with regard to your own senses.

[...]

Finally, when you cast a spell with a range of touch, your familiar can deliver the spell as if it had cast the spell. Your familiar must be within 100 feet of you, and it must use its reaction to deliver the spell when you cast it. If the spell requires an attack roll, you use your attack modifier for the roll.

—Find Familiar, Player's Handbook, pg. 240

Answer 1

Probably Not; but the rules don't expressly say, and it's ultimately a DM's call

The only claim to how Interplanar Distances should be calculated comes from a tweet made by Jeremy Crawford, lead designer of 5th Edition:

If two people are on different planes of existence, they are infinitely far away from each other. For example, if I'm on the Material Plane and you're on the Ethereal Plane, we're not within 30 feet of each other.

—Jeremy Crawford, Twitter, 2019-02-21, 9:10PM CST

But aside from the fact that his tweets are no longer intended to represent "official rulings" for the game, there's also the bigger and more substantial problem that the scenario presented doesn't have any explicit portals between the two planes in question. Crawford's tweet is telling in the fact that it either assumes, for the sake of argument, that there doesn't exist a single portal anywhere in the prime material plane to/from the Ethereal plane; or that even if such portals exist, the distance would still be infinite. But it doesn't clarify which assumption he's making.

So there is certainly evidence that the "Rules as Intended" are that the distances are infinite, and that such Familiar/caster communication would not work, but without actual errata stating how distances ought to be calculated across interdimensional portals, this question ultimately comes down to a DM's ruling.

Answer 2

It depends on the portal (and your DM)

There is a wide variety of portals. The DMG describes this on page 45:

"Portal" is a general term for a stationary interplanar connection that links a specific location on one plane to a specific location on another. Some portals function like doorways, appearing as a clear window or a fog­ shrouded passage, and interplanar travel is as simple as stepping through the doorway. Other portals are locations - circles of standing stones, soaring towers, sailing ships, or even whole towns-that exist in multiple planes at once or flicker from one plane to another. Some are vortices, joining an Elemental Plane with a very similar location on the Material Plane, such as the heart of a volcano (leading to the Plane of Fire) or the depths of the ocean (to the Plane of Water).

So, it depends on the portal:

• If the portal were of the kind where you exist on both planes at once, such as a clearing in the woods, as long as you are there you would share the dimension with your familiar (even if they moved outside) at the same time, and there it would just be the normal distance, so you could communicate -- they are not really "on the other side".

• If the portal would be like a doorway, it will depend on its exact nature and on how the DM adjudicates it. They might rule that the distance works like normal distance, or they might rule that the portal still separates the two planes, and the distance is undefined (or infinite), and you only can be physically transported though it by physically moving through it.