I've seen this discussed in past threads @tinglish and unfortunately it's a situation that cannot be easily troubleshooted.
Maybe it's the phone clinging onto the current connection type thats delaying the process of swapping connection type and or confusing the phone meaning a way to clear the process is a soft reboot which you've been doing.
When I leave my house while on my Home Wi-Fi the connection will remain until I pass the boundaries of the Wi-Fi connection and then will swap to 4G or 4G+ , but there is a slight noticeable stutter for my Internet connection to continue. It's like a pause when I'm surfing.
i.e
I commute by Bus to work so I use the bus companies app to track the bus.
It shows an icon of a bus on a map that moves as the bus travels. When I leave home the icon moves but when I move out of Wi-Fi range the swap to 4G happens the bus stops on the map and then seconds later jumps on the map to it's then current position.
All that said I don't have to restart my phone.
What troubleshooting have you tried thus far ?
Daily Driver > Samsung Galaxy s²⁵ Ultra 512Gb ~ Titanium Black.
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