really bad lag streaming upstairs.... FritzBox 7490 down stairs... New wifi modem? What about Asus RT-AC88U

All

We have UFB atr home, Fritz Box 7490, all firmware up to date etc..

The UFB box and Fritz box are down stairs and in a office but the smart 4K TV and all other wifi devices suffer lag around the house especially upstairs.

So what shall we do? I am really not wanting to run LAN cables all over the house (that what my kids are say) but must be a better way.

Looking at purchasing Asus RT-AC88U?

Or extender / boosters, but i think this may suffer as well?

We really need to boost the wifi strength around the house and remove the streaming lag as much as possible..

Cheers

Stephen