Welcome to the forum and sorry to hear about your issue.
The 12V battery can be "boosted" by using the relay switch on the lower left side of the dash and that should solve all of your issues.
YouTube will show you how to use the manual key to get inside the vehicle and press the relay switch. The manual will also explain the procedure but I agree that a video is much easier to follow.

 

I don’t see the switch.
The initial problem was that I left the headlights on. I thought they were supposed to turn off automatically.

 

Roadside assistance just arrived and we’re good to go.

 

I like to be prepared.

1. Soon after I bought my Niro, I made sure I could remove the mechanical key from the fob. It was very tight. A little bit of Teflon lubricant solved that problem.
2. Once I had the key out, I looked in the manual for instructions on how to use it. They were clear, and I unlocked the in less than a minute.
   
3. Once I understood the procedure, I showed my wife how to do it.

 

Got it.

 

Leave your headlights on "auto" and they will turn off soon after you exit the car.
I am assuming that even if you leave them "on" they should turn off after 10 - 15 minutes but that is likely too long for the 12V battery.

 

No boost option on the 2023 Kia Niro Plug-In.
The same thing happened to me. Since I took the car from the dealer two years ago I noticed that the battery wasn’t doing well, so I’ve been charging it about once a week. Over the last month, I left it unattended and the car lost all power. I had to reach the battery by folding down the back seats and connecting a charger, which provided just enough current to open the back door. From that point, the battery recovered from 5V to 13.1V at a maximum of 75%, but as soon as I switch off the charger, it drops back to 25%. The battery can’t be fixed.

 

No boost option on the 2023 Kia Niro Plug-In.
The same thing happened to me. Since I took the car from the dealer two years ago I noticed that the battery wasn’t doing well, so I’ve been charging it about once a week. Over the last month, I left it unattended and the car lost all power. I had to reach the battery by folding down the back seats and connecting a charger, which provided just enough current to open the back door. From that point, the battery recovered from 5V to 13.1V at a maximum of 75%, but as soon as I switch off the charger, it drops back to 25%. The battery can’t be fixed.

This thread is for the HEV, which has the relay switch on the dash.
As you note, the PHEV has a 12V battery in the rear cargo area.
I would connect a smart trickle charger and leave it connected for 24+ hrs and then see if the battery responds positively.
If not, the only option will be a new battery.

 

Ok. It’s been on for several 12-hour periods but isn’t holding a charge. I already have an appointment to get it replaced

 

I emphasize "smart" trickle charger as it will not overcharge the battery and will even reduce the sulfation effect that often occurs with 12V lead acid batteries.
Leave it plugged in until the green light illuminates.

 

I first connected the red charger at 4 A and then at 8 A. This was able to recover the battery from 5 V to 12 V at 100%. Then I connected the blue trickle charger, which only recognizes batteries above 10 V and can be left connected permanently, but did not go above 75% after hours and hours of charge and does not hold it. Anyway, once they put in the new one, I’ll still need to keep an eye on it regularly.

 

I strongly recommend you purchase a smart charger and connect it every 2 - 3 months, overnight, after you replace your current battery.
The charger you are using is not a smart charger.

 

Two chargers and a booster. I thought that was enough. It’s true that these two chargers do their job of bringing the battery up to the proper level, but they’re already a few years old and may be missing some useful features, such as proper SOH diagnostics or low-voltage recovery,... I’m already looking into smart chargers to upgrade my maintenance setup.