Serious Wifi calling issues/Receive Link Speed degrades to 6Mbps?

Since "upgrading" our speed to 300Mbps and installing the new XB6 gateway last October, we are experiencing three major problems.  We have contacted Xfinity at least 15 times, during which numerous recommended "fixes" were suggested (including replacement of SIM cards in both phones, bouncing gateway and phones/networks numerous times, and the recent onsite visit of a tech with replacement of the gateway device).  None of these fixes have helped.  I have done extensive research both on the Knowledge Base and AI-assisted internet, and have performed many tests trying to isolate the issue.  

The three major issues are:

1. MAJOR WIFI CALLING RELIABILITY ISSUES ON TWO PHONES (iPhone + Motog5G):

• Wifi calls: dropped mid-call

• Wifi calls: other party cannot hear us mid-call

• Wifi calls: not ringing, going directly to voicemail

• Getting Voice "mailbox full" message, though mailbox empty

• Voice messages delayed (hours to days) appearing in voicemail.

2. ONLY GETTING LTE SIGNAL IN 5G AREA

• WiFi calling failing over to LTE, not 5G, though we are in a 5G area

• Long delays (mins to hours) in SMS transmission to other parties.

• Unable to obtain any wireless signal at all for about 25% of the time.

3. 1 SECOND INTERRUPTIONS IN SOUND AND VIDEO STREAMING EVERY 15 SECs

This occurs 99% of the time on both iPhone and Android phones, but TVs only occasionally have problems.  It occurs for any video stream (Youtube, Facebook, TilTok, etc).

I checked and found that although Xfinity reports our internet speed is "EXCELLENT", our Wifi network speed is woefully poor. It appears the Router keeps dropping down modulation to the lowest OFDM rate of 6Mbps for seconds at a time, every 15 seconds:

• On Moto g 5G under Settings > Network & Internet > Internet > [My 5G Network name]:

Scrolling to the bottom of the page, the Receive Link Speed is continually switching from 390Mbps  to 6 Mbps (for several seconds) to 390Mbps to 6 Mbps (for several seconds) to 433 Mbps and so forth.

• Further tests/config changes have isolated the WiFi downstream RF impairment to the XB6 gateway router. Most changes to the router configuration that could correct the issue are disallowed in the firmware via the admin interface. Xfinity's app does not allow changing these advanced settings either. 

The tests/ workarounds I have implemented to isolate the problem include:

• Locking 5g and 2.4h networks using separate network names to disallow connection of IoT/ lesser quality devices to the 5g.

• Disabling Automatic Band Selection and manually setting bands on 5g to Channel 36 and on 2.4g to Channel 1 (to try to minimize band steering instability and DFS channel interference).

• Manually setting Channel widths (5g > 20-40 and 2.4 g also 20-40) to lessen channel overlap while not degrading signal too much.

• Disabling Xfinity Hotspot sharing.

The industry recommended workarounds cannot be made due to the admin interface being firmware-disabled:

• Disabling WWM Power Save

• Disabling Airtime Fairness

• Using 802.11ac or ax only

• Disabling IPv6 Network altogether to eliminate IP version contention (though I haven't yet tried manually setting IPv6 to unusable values) 

• Disabling Smart/Fast Roaming via 802.11k/v/r 

• Verifying setting of Eco/Transmit Power does not degrade RF signal.

In short, I suspect the XB6 is experiencing firmware-driven WMM power save or client idle throttling, along with DFS Bandwidth instability, or some other firmware setting that interferes with a reasonably stable receive link speed.

We don't want to have to have to bridge the XB6 and use our own equipment to try to obtain a good signal.  We can answer all your questions and can provide:

• Screen/video captures showing Receive Link Speed degrading to 6Mbps on my device while it is streaming video (while Xfinity speed test shows "excellent" connectivity).

• Screenshots of the huge number CM codeword errors as evidence of signal degradation:

Channel ID: 36

Unerrorred codewords 2795026067

Correctable codewords 1611094933

Uncorrectable codewords 609

Total codewords 4406121609

(this calculates to 63.44% Unerrored Codewords, a miserable QoS)

Can you help before we decide to abandon Xfinity?