Same issue MacBook Pro (13-inch, 2016, Two Thunderbolt 3 ports)

Connect your phone to 2.4 GHz Wi-Fi and it should work "If you have Apple Watch Series 6, you can connect to 2.4 GHz or 5 GHz Wi-Fi networks. Apple Watch Series 5 and earlier, and Apple Watch SE, can connect only to 2.4 GHz Wi-Fi networks"