Yea, duplicate IP addresses lead to some funny business. Toss a coin to see if a network packet will arrive basically.

The solution is to adjust the DHCP range or use static DHCP on the router. The latter just means that the router will assign the same IP to the specified computer every time.

Wait, smart bulbs run rogue dhcp servers now?

You need some .well-known url redirects for proper iOS support. Ironically, Nextcloud describes this quite well. You should be able to adapt this to your setup: https://help.nextcloud.com/t/properly-resolving-well-known-caldav-or-well-known-carddav/82617

I’m not sure if this is your exact problem but it’s worth investigating.