I had a similar issue, but only when using VPN. I had to change the mtu to 1280 and speeds are back to normal.
When you calculated the MTU, the modem probably was not set to 1500. Therefore the value you got through pinging was not accurate.
Although you see the mtu unset (blank) from the webpage, when you ssh into the box and run ifconfig you will see the actual value set on the wan interface(rmnet_mhi0); I have flashed few different firmware versions and found out that the value was variably set by GL engineers! 1428, 1472, and 1500.
I would advise you to set the MTU again to 1500 then repeat the pinging process to find the most accurate value.
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Another 1428 here. On Verizon.
I'm not certain the MTU actually fixed anything on my end, I did see timeouts at 1500 (1472) but not at 1428 (1400). I set it here by the way which is not readily documented because the LuCi interface doesn't support setting it. Let me know if that seems right?
What I eventually found is installing some QOS (cake + cake-autorate since I am on LTE) and disabling the router's "Network Acceleration" helped so far, my problem was I would get high ping and dropped packets under load which I'm guessing is related to my Zooom delay but I'm still testing this.
I also didn't know about this buffer bloat website which seems helpful for fixing Zoom peformance specifically (or any application that rely's more on latency) https://www.waveform.com/tools/bufferbloat