It's possible that the issue with your Nest thermostat is related to the battery, but it's also possible that there's another issue at play.
If you haven't already, try recharging the battery first to see if that resolves the issue. If the battery is low, the Nest thermostat may not function properly. You can do this by removing the Nest thermostat from the wall and plugging it into a USB charger using a micro-USB cable. It may take a few hours to fully charge.
If recharging the battery doesn't work, then replacing the battery may be necessary. It's true that replacing the battery can be difficult, especially if you're not familiar with the process. If you're not comfortable doing it yourself, it may be best to hire a professional to replace the battery for you.
As for syncing or resetting things when you return from vacation, it's possible that you may need to re-connect the Nest thermostat to your Wi-Fi network or re-register it with your Nest account if the battery has been completely drained. However, this should be a straightforward process that can be done through the Nest app. Just make sure to have your Wi-Fi network information and Nest account login credentials handy.
