Pairing with new iPhone

I’ve been trying to pair my Apple Watch with a new iPhone after not using it for quite some time. I no longer have the old iPhone it was paired to. I’ve already done a hard reset and erased all the content on the watch. It keeps getting stuck in an endless cycle of unpairing, downloading the software update, and re-pairing. The pair fails every time and it starts again from the beginning. It is fully charged and plugged in. Any ideas how to get this watch working again?