iPhone XS Max Weak 5GHz WIFI Reception

For more empirical Wifi Data you can download the Airport Utility app (no need to actually have an Apple Airport), and follow the following instructions from https://support.apple.com/en-ca/HT203068 to run the scanner utility from your XS and other iOS devices at the same location to get comparative data.
The Wi-Fi scanner in AirPort Utility
Apple’s AirPort Utility includes a Wi-Fi scanner that logs the client’s view of the network. Administrators can use it to validate the client’s view of the network at a specific location.
For accurate results, use the Wi-Fi scanner on a dedicated device that’s the same model as the iOS client.
On your iOS device, go to Settings > AirPort Utility to turn on the Wi-Fi scanner.

Next, open Airport Utility and tap Wi-Fi Scan.

By default, Wi-Fi Scanner runs continuously. Use the slider to set a scan duration of up to 60 seconds.

To start the scan, tap Scan. AirPort Utility lists all the SSIDs that it finds. This includes hidden networks, which appear as "Network name unavailable."

The AirPort Utility scans all available bands at four-second intervals. Enterprise networks that have multiple access points are grouped by BSSID. The scanner shows information about:

• SSID
• BSSID
• Last RSSI
• Channel
• Last Time Found

To view a trace log of the scan results for an SSID and BSSID, tap the SSID:

The trace log shows the date and time of the scan, along with the channel and RSSI.
After the scan completes, you can share the results. Just tap the share icon (), then choose one of these options:

• AirDrop
• Message
• Mail
• Copy

AirPort Utility sends the results as a comma-separated list:
SSID, BSS, RSSI, Channel, time
"ACES", "18:64:723:E9:40", "-57", "11", "12:02:03 PM"
"Cuba", "F8:1EF:F9:56:BC", "-53", "149", "12:02:03 PM"
"ACES", "18:64:723:E9:50", "-63", "149", "12:02:03 PM"
"Cuba", "F8:1EF:F9:56:BB", "-69", "11", "12:02:03 PM"
"ACES", "18:64:723:E9:40", "-67", "11", "12:02:07 PM"
The first line is a column header that shows the SSID, BSS, RSSI, Channel, and date fields. To analyze or chart the results, import the list into a spreadsheet or other tool.