Your router / DHCP server is the authority that hands out IP addresses to the devices in your network.
If you set an IP address on the device itself, your router does not know and can assign that same IP address to another device. And then one thing is sure; your Homey will respond even worse 
Your ISP only hands out the Public IP address of your router. If that changes, it does not affect your internal networks IP addresses.
If you change ISP and your router belongs to the ISP, you probably would have to redo the fixed Homey IP.
But, maybe even more important: what are you trying to solve? As for me, dynamic assigned IP works just fine on Homey.
