“None” was ruled out by making it a red line to leave the customs union and single market even though that was originally something quite of leave campaigners thought was a silly idea. And it was ruled in by the WA backstop if the UK couldn’t figure out how to leave the Single Market without violating the Good Friday Agreement.
NI/RoI seems to violate the Good Friday Agreement. People argue about if it really does, but ultimately what matters is if the situation no longer satisfies the parties involved, and Sinn Fein says it doesn’t.
Irish Sea enrages the DUP because they want to be identical to the UK mainland (except for abortion laws and gay marriage etc).
It’s a mess.
One “solution” I’ve seen leavers suggest is for RoI to re-join the UK. The is generally met with derision by RoI commentators.
The Irish backstop was the EU's solution, not the UK's. The talking point you're probably thinking of is that the UK-wide backstop was the UK's doing, which is technically true because the EU wanted the backstop to apply only to Northern Ireland - the UK negotiators objected because this would make a huge mess of the Good Friday Agreement (not to mention pissing off the DUP). I can't say I blame you for getting this wrong, since the whole point of this talking point was to leave people with the false impression that the backstop was the UK's doing.