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Switching from a barely working S21. Any tips what I should set up on the new phone?

Switching from a barely working S21. Any tips what I should set up on the new phone?
Country: Poland
Cat age: 15
Cat sex: male, neutered
Vet info: we haven't contacted them yet but are planning to. We can afford vet care.
My 15 year old indoor-outdoor cat is constantly going missing and returning for the past two months. We never had similar problems with him, he always came back for the night / showed up the following morning. He is neutered.
Mid June he was watching me cut a tree in the garden and that was the last time I saw him during the first missing period. It was the roughest weather this whole year where I live. 38+ degrees Celsius and no rain for a couple weeks. The family was also incomplete because my father was in hospital (idk if it matters). He came back after exactly two weeks and behaved like nothing happened. Went straight to the bowl to eat. Our other cat (4, neutered) didn't seem surprised by his return. Next day we found a wound on his neck, but it was nearly healed and didn't hurt (no reaction to touching). I suspect the wound was why he hid from us that long. We sadly didn't have time to get him to the vet then, but the wound healed properly by itself pretty fast. We are currently booking a visit for a checkup and to get him chipped.
A month later me and mom went on vacation and cats stayed with my dad. Once again, the family was incomplete and it was another exceptionally hot week. After a couple days the cat went missing once again. After two weeks he quietly showed up in the living room. I haven't found any wounds this time, but I also didn't have much time to look because he was gone in like 15 minutes.
Showed up 3 days later. No wounds. Currently laying next to me and purring, but doesn't really want to cuddle. What do you think might be the reason? What do we do next? I suspect he might have found a new home or is starting to develop dementia.
TL;DR: My 15yo cat went from coming home every night to vanishing for 2-week stretches over the past two months. The missing periods overlap with heat waves, some family members travelling and a neck wound.