JohnAtl has the right idea. Have the HomePod sign on to one of the two networks (2.4 and 5Ghz) and then make sure the iPad signs on to it as well. If you choose the 2.4Ghz network, then you can tell the iPad to forget the 5Ghz network in Settings app. In my case I didn’t have to do that. They both are on the 2.4Ghz network (see below).
Don’t eliminate the 5ghz network from the router, it is useful. The general rule is that 5Ghz is faster than 2.4 but 2.4 has longer reach. Of course, mileage varies depending on your house, interference from appliances, the number of connecting devices and competition from neighboring networks.
WiFi networking is a big mess. From what I hear, mesh networks are a big improvement over traditional networks. Unlike with some people, my devices almost always sign on to the 2.4Ghz network even though the AirPort router is is set to choose the best network and we (the devices and the router) are all in the same room only a few feet apart. I have the loneliest 5Ghz network!