Rather than trying to use presence detection, I would use a keypad at the back door. There are two that work well with SmartThings, one Zigbee and 1 zwave. The Z wave device also comes with an NFC tag that you can just touch to the keypad if you don’t want your neighbor to have to remember a code.
If your weather is severe, you may want to put the keypad inside a plastic box to shelter it.
Both these options are discussed in the following thread. ( this is a clickable link)
The zwave model runs around $59, but also includes the key tag. ( shop around, price on this one varies a lot.) Home Controls and NewEgg usually have it at a lower price than Amazon.
The zigbee model lists for around $45, but is often on sale at Lowe’s.