Marshmallow: Changing and connecting to an APN
Thanks for the insight of user157947 though, I found something that helped:

I set all the Bearers in all my other APNs to something jibberish (not applicable in my country, like IS95A). I haven't seen this feature in Lolipop though (it was only LTE iirc)

To set it back to the original APN, set the Bearer to Unspecified then set your custom APN to IS95A.

I think I can also use this feature as well to force a custom APN on LTE on Marshmallow.