By Ping test method, packet fragment happens after packet size is more than 1452.set it at 1460 and check .
If you want to find the optimal mtu value check this thread :
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Optimal MTU value | BSNL Bharat Fiber Broadband
Many users complain that some websites do not open, and usual suggestion is to change the MTU values in modem. Applications are also recommended for trying different values. tcp/ip optimizer from speedguide.net. Now the following is another method for MTU selection. General instruction for...broadband.forum
Its not happening on 1 Phone. Its happening on 2 mobiles (Samsung S20+ and OnePlus 7) and 1 Tablet (Samsung S6 Lite).@SVK probably something to do with your phone then. Try looking at the wifi log.
Run a malware scan as well.