DIAGNOSIS Most smart home devices are 2.4GHz-only, and they're notoriously picky. The Velop's combined SSID (or even the split 2.4GHz network) can sometimes use security protocols (like WPA3) or settings that these cheap chips don't like.
FIX In the Linksys app, go to Wi-Fi Settings. Ensure "Security Mode" is set to "WPA2/WPA3 Personal" or just "WPA2 Personal." Pure WPA3 will break many older devices. Also, temporarily turn OFF any "AP Isolation" or "Client Isolation" setting you might have enabled under Device Prioritization or Advanced settings.
IF THAT DIDN'T WORK For really stubborn devices, create a dedicated 2.4GHz guest network (with WPA2) and connect the smart devices to that. The simpler, slower guest network often works where the main one fails. Keep the guest network always on.
HEADS UP When adding a smart device, if the app asks for a password, some devices can't handle special characters. Use a password with only letters and numbers on your 2.4GHz network to rule that out.