DIAGNOSIS This is often a DHCP lease time issue or a conflict with 'Smart Connect' (which blends the 2.4GHz and 5GHz bands under one name). The device gets confused about which band to use or the router's IP address assignment gets stale.
FIX First, try splitting your bands. Log into the router admin page, go to Wireless > General. 1) Give your 2.4GHz and 5GHz networks distinct names (e.g., HomeWiFi_2G and HomeWiFi_5G). 2) Disable 'Smart Connect' if it's on. Apply settings. Connect your problem devices manually to the 5GHz network for best performance. This removes the band-steering variable.
IF THAT DIDN'T WORK Adjust the DHCP settings. Go to LAN > DHCP Server. Increase the 'Lease time' from the default (might be 86400 seconds) to something longer, like 604800 (one week). This tells devices they can keep their IP address longer, reducing renegotiation hiccups.
HEADS UP Some older IoT devices (smart plugs, lights) have terrible WiFi chips that struggle with modern routers. If it's only one type of device failing, you might need to create a separate, 2.4GHz-only network with a simpler name (no special characters) just for those gadgets.