A subnet IS NOT anything to do with resilience… period… they are very different things. A subnet is a network definition with a network address and an address or range of addresses. All routing on the internet uses subnets - even if simply /32, and just about all home networks are defined by at least one subnet - such as 192.168.1.0 /24
You my have a local configuration or setup using multiple networks with multiple accesses or some other local specific configuration - . but that is more to your configuration - and not subnets. Without a subnet no one could effectively do anything on the internet… or have a home IP network address range.
Local connections on your home network are not ‘reset’. You may see that the DHCP assignment memory gets reset if the DHCP server is power cycled - they are often put into home routers… however a good DHCP server should not ‘reset’ existing assignments if active.
Whether you use ethernet or wifi has no bearing on any of this.
‘Disappearing’ UPnP devices (where you are referring to an UPnP application not ‘seeing’ a device) is typically because of network product issues in handling specific multicast addresses - which can result in some poorer performing devices (switches, wifi access ports etc) failing to send SSDP query through to a device which is expecting it - which results in the device not responding and therefore ‘disappearing’
One workaround - not ideal - but can help with poor products - is to disable IGMP snooping if the capability is made available. This will typically blindly broadcast specific multicast addresses to all devices - irrespective of whether they are joined to a group or not. If you are seeing a difference between ethernet and wifi - then that does point to a poorer wifi access point implementation.