DIAGNOSIS This is typically a software glitch with the TV's network stack or a conflict with your router's settings, not a broken Wi-Fi card.
FIX Start with the TV. Go to Settings > Network. Forget your current network. Then restart the TV (Settings > My Fire TV > Restart). Once it boots, go back to Network, find your Wi-Fi, and re-enter the password. If that fails, try this sequence: 1) Unplug your router/modem for 30 seconds and plug it back in. 2) Unplug your Fire TV for 60 seconds and plug it back in. 3) Once both are fully booted, try connecting again.
IF THAT DIDN'T WORK Check your router settings. The Omni QLED can have trouble with certain security types. Log into your router and try temporarily changing your Wi-Fi security to WPA2-Personal (AES) only, disabling WPA3 or 'mixed' mode. Also, if you have a dual-band network (2.4GHz and 5GHz), try connecting to the 2.4GHz band—it's more stable for these TVs.
HEADS UP If you've recently changed your Wi-Fi password or router, you must 'Forget' the old network on the TV first. The TV will keep trying the old saved password forever otherwise.