You might consider 'The House on The Borderlands'the 1908 novel 'The House on The Borderlands' by William Hope Hodgson. That amazing story fulfils a lot of firsts. ItIt was written during the first Edwardian flirtations with what we would consider 'real SF' as opposed to the often politically-motivated satires of Wells. It also principally involves what today might be considered a 'soft spot' in the barriers between dimensions and times - offering the protagonist a view of a FAR future Earth which is inhabited by creatures so far removed from our own world they would be at home in Lovecraft's much later offerings.
A portal in every sense of the word.