5G connection dropping down to 2.4G automatically after few hours.

My Opal is being used as a router extender (in router mode for firewall and other purposes) and is connected too the living room main router. I have it connect it to it’s 5G which for the first hour or two, it will remain with 5G connections and speed. But after like 3 or 4 hours. Or Whenever I boot up a game, It drops the connection down to 2.4G (despite not adding it to my settings) and drop speed down to 60-70mbs. On my browser admin page, it’s still connected to 5G but on my mobile phone app, it clearly states it dropped to 2.4G as input. How do I prevent this and how do I lock it in 5G so this will not happen?

Do you have both SSIDs as the same Wireless Network name?