I just got a new WiiM Pro and running into the same issue with it, so much so, I can't even set it up. For me, it works fine on a direct ethernet connection, and I can get it connected to my router over WiFi but the app never is able to find it for some reason (both the app and WiiM are always on the same wireless network). From my router, I can see and confirm that the WiiM gets IP addresses and I can ping it so I know the device is able to connect and I was also able to port scan the WiiM so I can confirm it's services are accessible:
user@host ~ % ping 192.168.0.122
PING 192.168.0.122 (192.168.0.122): 56 data bytes
64 bytes from 192.168.0.122: icmp_seq=0 ttl=64 time=45.023 ms
64 bytes from 192.168.0.122: icmp_seq=1 ttl=64 time=50.906 ms
64 bytes from 192.168.0.122: icmp_seq=2 ttl=64 time=35.742 ms
^C
--- 192.168.0.122 ping statistics ---
3 packets transmitted, 3 packets received, 0.0% packet loss
round-trip min/avg/max/stddev = 35.742/43.890/50.906/6.242 ms
user@host ~ %
user@host ~ % nmap -p- 192.168.0.122
Starting Nmap 7.94 ( https://nmap.org ) at 2024-05-01 20:01 CDT
Nmap scan report for 192.168.0.122
Host is up (0.0040s latency).
Not shown: 65514 closed tcp ports (conn-refused)
PORT STATE SERVICE
443/tcp open https
2019/tcp open whosockami
5356/tcp open ms-smlbiz
7000/tcp open afs3-fileserver
8008/tcp open http
8009/tcp open ajp13
8012/tcp open unknown
8086/tcp open d-s-n
8443/tcp open https-alt
8819/tcp open unknown
8893/tcp open ddi-tcp-6
9000/tcp open cslistener
10001/tcp open scp-config
10101/tcp open ezmeeting-2
18008/tcp open unknown
18100/tcp open unknown
24012/tcp open unknown
25012/tcp open unknown
45727/tcp open unknown
49152/tcp open unknown
59152/tcp open unknown
Nmap done: 1 IP address (1 host up) scanned in 80.45 seconds
user@host ~ %
My router is a newer TP Link (BE19000 Tri-Band Wi-Fi 7 Router) so I have multiple radios 2.4GHz, 5GHz, and 6GHz all with the same SSID so clients can switch to their optimal/preferred speeds. I'm guessing something about this is the issue since I was able to set up a guest network locked on 2.4GHz radio that I was able to bootstrap the WiiM on (since I was able to break isolation from that network, my other devices can connect from the other radios without issue) so it's functioning without issue now. It's still a major pain since I'd rather the WiiM be able to leverage all of my network's bandwidth. I even have suuuuper old IoT clients that don't have any issue connecting so it just seems to be an odd issue inside the WiiM Home app for my situation.