guy invents metal rings that he can put his hand threw and it comes out of another ring.
Might not be what you are looking for.
Libby Left Her Penis Home by HoneyMoon
Science Fiction
Waking up late with a case of Morning Wood, Libby Edwards doesn't have time to waste taming the beast. Thinking fast, she uses a mini-portal to leave her erection home so she can wear her new dress without an unsightly tent. Before long she realizes she made a fundamental mistake. Her daughter Molly was home alone, with her erect penis!
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Tags: Romantic, NonConsensual, Hermaphrodite, Science Fiction, Incest, Mother, Daughter, First, Oral Sex, Masturbation, Water Sports, Pregnancy, Cream Pie, Size, Big Breasts, Workplace
Sex Contents: Much Sex
I think you are looking for "Micro Gates by PT Brainum"
https://storiesonline.net/a/pt-brainum
https://storiesonline.net/s/17944/micro-gates
Also, Elements of Power is also fairly similar in style... you would enjoy that one, too, if you liked Micro Gates
That story has a similar idea as a series of books by Laurence Dahners, the Ell Donsaii series, the the gates (or ports as they are called in the books) don't enter until the 3rd or 4th book in the series. I read them on Amazon Kindle Unlimited. There are about 20 books in the series. I found the first 5 or 6 were pretty good, the next few were OK, but once he got up past 12 or 13, I sort of lost interest and haven't read the last 5 or 6.