Casting spells that your character typically couldn't

Question

So I have a warlock that is interested in trying to set up a "pocket dimension" if you will, using Demiplane. The issue is that if I want to be in that plane for extended periods I need to set up either (from my understanding) a permanent teleportation circle inside and outside of the demiplane so I can travel between them at will (given I have some way to cast the spell) or set up a gate so I can travel there.

The question, pertaining to teleportation circle more than gate (being that it's my preference) is that as a warlock, is there any way I can cast it without spending most my levels in a different class. (e.g. though the use of a scroll or a spellbook with teleportation circle enchanted into it or something) Or do I just need a pocket sorcerer with me at all times to allow movement?

Again this is just so I can end the spell so the door disappears and not get trapped.

Answer 1

Teleportation Circle will not help you

Teleportation circle allows you to go to a circle "that is on the same plane of existence as you." A demiplane is not on the same plane of existence.

Plane Shift is the answer

You and up to eight willing creatures who link hands in a circle are transported to a different plane of existence.

... if you know the sigil sequence of a teleportation circle on another plane of existence, this spell can take you to that circle.

It's a 7th level warlock spell.