That Asus router comes with unnecessary technology that most other devices do not use. the 'ax' line of Wifi bands. So no devices, especially your phone can utilize it. Based on location and obstacles around the house like thick walls and even placement of router beside a very strong electronic equipment like a TV and microwave can make it not able to cover the whole house.
Your best bet now since you already have this router, is you can buy or reuse an old Asus router, update the firmware and use the Asus Mesh AI functionality that makes your second router act like a mesh device along with your main router.
So just place the old/cheaper router somewhere in the middle of the house so the 2 routers can communicate and allow the second router to give you the same Wifi connection as the first one. Thus boosting the weaker area of the house.
The connection is the same as the first one. So your phone will automatically switch signals on its own. Plus, you don't need to register another Wifi on the phone. Cos, same network.
Either you use this method, or you have a long Lan cable from the Modem to your main router which you must then place in a better location in your house. It's up to you. Both methods should be better than your current one.