That is a puzzler. But a couple of suggestions. First, is the password for the current network that you want to use the same as the old network? If so try changing it. If it's the same it may somehow be leading your device to your current router even though you've changed the name of the network.
Tthe other idea concerns iCloud. Any network that you've connected to by any device logged into your iCloud account including your Mac, if you have one, will appear in your list of networks. Even networks I joined years ago with different computers and devices will appear in my list. So if you have a computer logged into your iCloud account go into System Preference/Network and under the Advanced tab/WiFi you will see a list of all those networks. Use the minus sign to delete all those you have no use for including the one you are having the issue with.
Keep in mind if you ever join one of those again on any device it will reappear. At one time I replaced my router, an Apple Time Capsule, with a new one and I made a slight change in the network name that I wanted to use, basically it only involved a punctuation mark, and the change was unintentional. However my devices continued to use the name of the original network and connected to it because the passwords were the same. I didn't realize this for a while because the connection was automatic. But when I saw that I had 2 similar networks in my list I was able to delete the old one and make my devices connect to the (slightly) new one.