Strange observation: iOS and Static Lease = unstable iPhone

One of my friends iPhones (model 6s, still on iOS 9.02) has the last months been behaving strangely every time it connected to my Turris Omnia wireless network. Most times the phone would crash immediately it connected to the WIFI, other times it would crash shortly thereafter.
This evening we experimented with it, having the phone connected to the 4G network and wifi disabled - everything worked for an hour+. Then enabling the wifi and within seconds the iPhone crashed.

Searching for clues online, there were an article that mentioned that especially the iPhone used a fast wakeup method, where it attempt to send on a previous DHCP lease before verifying if it was still valid. Apparently something that used to make routers crash - but should be fixed in modern routers nowadays.
It made me think that I should attempt to set up a Static Lease for the iPhone, and see if that changed anything.
… But when about to setup the static lease, I realized that we had already set up a static lease for some testing several months back - but we had forgotten everything about it.
So I tried to remove the static lease, and suddenly the phone no longer crashes when connecting to the network.

I then tried testing another iPhone (Model 6s, but with 10.3.2), adding it to the DHCP static lease. It didn’t crash, but after a while it switched away from wifi and preferred the 4G network. Only restarting the phone made it connect to the wifi again - for a while.

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