Issue with key fob, wondering if it's the battery?

Hi y'all. I have a 2024 Mitsubishi outlander that I've had for almost a year now and I'm the second owner (it had been used as a fleet car before I bought it from Carvana).

On Wednesday morning when I went out to leave for work for the day my key fob would not unlock the car. The red light on the fob lit up but it wouldn't unlock. So I had to use the key to manually unlock my driver's door-which proceeded to set off my alarm that I couldn't turn off until I started the car. I figured then maybe the battery was starting to go-but then the rest of the day and up until today, Saturday, the fob has been working fine like nothing is wrong. I've been able to lock and unlock and what not.

This morning however, the same thing as before- leaving for work-key fob wouldn't unlock, I had to use the key to unlock the door, the alarm went off until I got the car started.

So should I assume this is a battery issue? Something else? Also why the hell is my alarm going off when I use the actual key to unlock the car door?

Is the battery in the fob easy to replace or am I going to have to take it to a dealer and see if they can help me? In the past when I had a van it was pretty easy to replace the battery but I haven't looked too closely at this key yet to see if this is going to be a pain in the ass or not.

TL;Dr: key fob is working inconsistently, need to know if we think it's the battery and also why my alarm goes off on the car when I use the key to unlock it.

TIA.