iPhone Xs won't update for Apple Watch Ultra

Xs needs to be 26.1 to connect to apple watch! I needed to buy the cheapest iphone I could get to work with my apple watch. I looked at the listings on this site and it said iphone 8 or later running ios16 or later for an Ultra running os9 or later. So I have an Xs running 18.7.2 and an Ultra running os 20.1. According to apple this should work, but it doesn't. It appears I need to upgrade my Xs to 20.1 but this is impossible. Great! So now I have an unusable Ultra and a useless iphone, as I only wanted it to be able to run my Ultra. I use android on a Samsung fold 7, but I hate the samsung watch and wanted my old Ultra back.