Possible MTU issue with 4G LTE WAN connection. Websites load slowly or partially, but pings are fine.

Main problem: Pings to google.com and any internet address are fine. Websites load sporadically -- sometimes fully and instantly, sometimes partially, and sometimes not all. Speedtest.net app results are great at times, and other times will fail the latency test.

My WAN connection is a 4G LTE connection using a Verizon Wireless Novatel 7730L hotspot tethered by USB to a GL-AR300M router with OpenWRT 18.06.0-rc1 r7090-d2aa3a1b62 installed. I have also tried using a different router, an Asus RT-AC1900P, with essentially the same experience.

I'm suspecting this might be an MTU issue.

In OpenWRT, I have changed the MTU for both the LAN and the Tethering interfaces to 1428. I have also enabled MSS clamping on both the LAN and WAN firewall zones.

I have used the mturoute.exe tool (found on another forum) to do tests to both Google and to my gateway. The result for Google says the path MTU is 1428, and for my gateway it says the path MTU is 1500.

Any ideas on what to check?