[X Style (Pure) Q&A, Help & Troubleshooting] ResistanceIsFutile:
As [MENTION=2444885]aybarrap1[/MENTION] has pointed out already, you are running a stock Nougat baseband,

Meaning you've also upgraded fully to the Nougat OS on your device.

Possibly, for someone new to the XT1575 on Nougat OS it's a good place to stay and get acquainted.

There's nothing wrong with Nougat. Root and TWRP are possible, as well advancing us further than what the Marshmallow OS offered.

I can understand Custom Rom wise, your wanting to "Is there a way to get The Marshmallow BaseBand?" but, consider that's just water under the bridge now - never to be the same for you. In anything you'd do to achieve this would just put your device in a Mixed-State, and where an advanced user would be better adapted of dealing with that.

That's why in my 1st. post to you I was trying to encourage you to read more, let it go awhile and sink-in, and read some more getting a mental picture as you understand the depth of everything related.

Or simply sell off your now XT1575 Nougat OS device and what to look for in a stock-locked bootloader of a Marshmallow OS XT1575 device and stay there not taking an OTA of Nougat.

Even in some Custom Rom's built for a Stock Nougat Kernel and Modem there have been changes related to the TWRP version needed to be used of them.

Here gives abit about that information.

I use the NucleaROM on one XT1575 and kind of bias towards it as I've stayed with it in my first flashing of a custom rom when OTAing up to Nougat that is after failing with LOS of the jumbled up thread for clark it uses.

[ROM][OFFICIAL] LineageOS 14.1 Nightlies for the Moto X Pure 2015 (clark)

Trying to decipher this LOS thread posts of Official versus Unofficial builds, versus using Modems of MM and Nougat and talking about LOS 14.1 and 15.1 builds along the way and all the different involved posts of each Official - Unofficial unlabeled subjects there of all the posts.

It's no place for an inexperienced user to go and easily understand it.

These are the posts https://xdaforums.com/showpost.php?p=75763876&postcount=1303%22]https://xdaforums.com/showpost.php?p=75763876&postcount=1303 that clearly pointed out the specific information in details that helped me to know of the differences to the posts while I was active in that thread.

I like the NucleaROM because it's being based on LOS, then I can and do - do it without Google for the most part. I've not flashed any g-apps what so ever, play store is not a concern for me to use - there are other less invasive places that don't have the requirements Googles PS imposes. (F-Droid)

Even running a root file explorer I don't see anything Google related - with the way I've installed NR :)

Besides running an apk through Virus Total gives me peace of mind.

Android is by Google and that's were G stops with me...

NucleaROM - (root - g-apps), all are optional for the two NucleaROM builds out.

Note:It's the wakeblock app or it's folder that is mandatory to do something with in NucleaROM.

In disabling it's app in phone settings (Apps) after 1st. boot, or removing it's folder in NucleaROM's root directory via TWRP before your very 1st boot into OS where you initially setup the OS.

Did any of this help?