Ping intermittently lost packets

I’m using GL-MT300N-V2 to tethering my Android phone. The tethering apparently works but intermittently pinging a server (like ping would lose packets and the mwan3 would consider the tethering is offline. The internet on the phone is perfectly fine when the router loses packets. Any suggestions on how to pinpoint the cause?

Here’s what’s weird. I did two ping tests simultaneously, one from the router via ssh and one from a PC connected to the router. Ping from the router lost packets while ping from the PC doesn’t. Any suggestions?

Apparently my phone’s service provider is blocking ping. Most of the time when I got 100% packet loss by ping, curl would still work fine. How can I let mwan3 use something other than ping to track connectivity?

From a WIndows PC, you could try hrPing [Ping Utility hrPING v5.07 - cFos Software]
It can do UDP instead of ICMP.