Coverage and capacity are different
Coverage asks whether a signal reaches an area. Capacity asks whether the network can serve the number of clients and amount of traffic there. Adding an access point can improve capacity but can also create interference if channels and power are planned poorly.
Use the band deliberately
2.4 GHz generally travels farther but has fewer non-overlapping channels. 5 GHz offers more channels and commonly higher capacity. 6 GHz can provide clean spectrum for compatible clients. Channel width, regulatory limits and neighboring networks affect the decision.
SSID, BSSID and roaming
The SSID is the network name users select. The BSSID identifies a specific radio, usually by its MAC address. An extended service set can present one SSID across multiple access points. A client may roam between radios, but roaming quality depends on design and client behavior.
Troubleshoot in layers
Check whether the client sees the SSID, associates, authenticates and receives an address. Then test the gateway and a remote service. A captive portal, wrong passphrase, weak signal, channel congestion, DHCP failure and DNS failure produce different evidence.