keen to improve coverage of Apple Model A1521 router

Is it currently configured as main router, bridged or wireless extended from another airport??

Ensure all connections are gigabit ethernet. ie No airport express are involved in the network. Or any router or switch that is old low speed device. Make absolutely sure gigabit connections can actually achieve gigabit speeds.

If you must use wireless.. use only one hop.. ie DO NOT EXTEND BY WIRELESS EVER.

Make sure you can select 5ghz manually if the client does not select it.. to do this you will need to use a different name for the 5ghz band.

Max link speed on 2.4ghz is 217Mbps.. Max link speed on 5ghz is 1300Mbps. see the difference.

The problem is the clients do not join the fastest band.. they join the band with the best signal.. mostly that will mean 2.4ghz which has much better transmission through solid objects like walls and furniture and self propelled meat bags.

Make sure the airport is placed in the best location close to the clients and high up in the room so there is as little solid materials to block signal as possible.

Small changes of location can have big effects on speed if metal objects are blocking the direct path. Metal is used in many places in the house.. e.g. insulation, plaster backing, mirrors, tinted glass, plumbing, etc.

A single airport will be unlikely to cover the whole of a multi storey house.. you need a wireless access point on every floor.. or a long thin house needs points in the middle and each end ..

Floors are great at blocking signal as they are much more solid than dry walls which are designed to divide rooms but contain as little material as possible. Homes made out of solid concrete and brick are the worst for wifi.. it is very difficult to penetrate more solid material than timber and plasterboard.