I have the same issue. It doesn't see 5GHz SSID at all. It does see 2.4 GHz and connects to it. But even with 2.4GHz connection is not stable. It disconnects often with 2.4GHz SSID disappearing for some time.
Wi-Fi router is ASUS RT-AC88U. Control channels set to auto for both bands.
Everything else at home works with both bands perfectly fine.
LibreElec 9.1.501