Hi @CTYinNC,
Welcome to the TP-Link Community Forum!
OneMesh connectivity from one router to another is not yet supported, which is why you don't see the Archer A7 as a satellite option when attempting to set up OneMesh via your Archer AX55. Currently, only one router can be in a OneMesh network while supporting satellite devices such as Range Extenders.
We would recommend EasyMesh or our Deco Mesh solutions for this type of router-to-router mesh. For a list of EasyMesh compatible TP-Link routers, please take a look here.
For a general overview of how EasyMesh works with TP-Link products, check this page out.