Try checking basics first, if all antennas are attached properly, if other devices are unable to see 5G SSID as well. Doubt that firmware is the problem, you can always try testing it by downgrading it with the same firmware you were using before.
There can be many other causes for WiFi signal interference, like unsupported, congested channels, DFS, and physical factors. By installing an application like Wi-Fi analyzer or Wi-Fi channel scanner on your mobile device you can scan the surrounding Wi-Fi signals and avoid congested channels.
Try to avoid using DFS (Dynamic Frequency Selection) channels. Avoid channels 52-144 and choose between 34-48 or 149-161 instead. As DFS channels can automatically switch to other channels with lower interference, they may cause a brief disconnection during the switch.
5 GHz networks are more suitable for open spaces. Obstacles (such as cement walls, iron, and glass) between your device and router can greatly reduce the transmission speeds and signal strength. Since RUTX11 is a dual-band router, try using 2.4GHz and 5GHz at the same time, to lower the risk of losing the signal.
If nothing helps, try uploading 2 troubleshoot files: one with older firmware where 5ghz WiFi worked properly, and one with the newest, where you have issues. (Troubleshoot file can be found on Router UI -> System -> Administration -> Troubleshoot, and must be downloaded once the issue is present. More info on the links below)