Hi,
We use a smart lock, and then our back-up is a lock-box with a key inside. The lock box is similar to one real estate agents use - if your smart lock runs out of battery or isn't working for whatever other reason, you give the guest a code to unlock the lock-box with a key inside. Our lock box is mounted next to the front door.
We use Guesty for hosts. Guesty syncs with our smart lock, so we can see when the battery is low so that we can have our cleaning service change the batteries before it dies. Guesty automatically generates a code for our guests. I have an automated message with check-in instructions that gets sent to the guest with their auto-generated smart lock code 2 days prior - I don't have to put in the code, it's all automatic. We set it up to be the last four digits of their phone number.
We never have to change the lock box code because we only give it out if it's needed. This has only happened once in 2 years.
Hope that helps:)