Not 100% sure on ASTRO Portable CPS, but in ASTRO25 Portable CPS, after making a new personality, you need to look under the TX tab of the personality and make it digital or ASTRO voice. Then once you tie that personality to a mode, you should be able to select a NAC. To have both digital and Analogue modes, you need at minimum, 2 personalities. One set for analogue voice, the other set for either digital voice or ASTRO voice. ASTRO is what Motorola calls P25 voice.
I have never used/owned/programmed a XTS 3000 or ASTRO Saber, bit have a couple XTS2500 portables.
In all honesty, the XTS2500 or XTS5000 are much better choices, not just from an accessory point of view, but a programming point of view as well. You can use a proper USB cable and a modern windows computer. No need to piss around with trying to find compatible hardware. No need for a RIB(a 'Ribless' cable still has a level converter that emulates emulates RIB built into the cable) or a hardware serial port/USB<->serial converter and that associated headache. Not all USB<->serial converters are created equal.
Glad it's working, but the radio is 20 years past EOL. Its got a best before date you can't predict. Even sitting on a desk just receiving.