how do I remotely access my voicemail

Quick start guide here:

https://att.vtp-media.com/products/DLP/DLP73X90/DLP73X90.pdf

implies the feature exists, but doesn't tell you how to use it.

Full guide here... oops, no, just a different copy of the quick start guide:

https://att.vtp-media.com/products/DLP/DLP73X90/DLP73X90_QSG_i1.0_20210810.pdf

Full guide HERE:

https://cdn-media-att.vtp-media.com/products/DLP/DLP73X90/DLP73X90_WEBCIB_i1.0_20211018.pdf

Logical page 151, physical page 158, tells you how to set up the remote access code

Logical page 161, physical page 168, explains how to use the remote access to your phone's built in answering machine feature.

Moving to the home phone section

What do you mean by "connected to a cell answering system? That makes no sense unless you're talking about having the phone forwarded to your cell phone.