Marshmallow: Changing and connecting to an APN

Okay, before anyone can downvote this or anything silly, I need to clarify a few things:

1. This is for Android 6.0 (Marshmallow)


2. I'm using a FlashFire-Rooted Cherry Mobile One (technically called Android One Sprout8)


3. Prior to upgrading into Marshmallow, I am able to do these steps.


4. This is not a duplicate of this particular ticket or this one, please.

Scenario

I want to use a certain APN (for a certain cellular network) which does not connect to the internet without a corresponding VPN. To do that, in the previous versions of Android, I simply create an APN with the corresponding configurations, switch to it by tapping the radio button, then the Android system connects to that APN, even if it can't connect to the common internet.

In Marshmallow, (or at least the stock ROM I flashed?) this doesn't work, and the it connects to the default network APN. It's similar to how my other phone, ZenFone 2, works, but with LTE (always defaulting to the nearest working APN; otherwise HSPA+ works.)

Is there some way to force the APN into this system? But I wouldn't want to downright delete my old APNs, as I still would want to connect to them when needed.