How does MTU size effect you?

MTU: 

A maximum transmission unit (MTU) is the largest size packet or frame, specified in octets (eight-bit bytes), that can be sent in a packet- or frame-based network such as the Internet. The Internet's Transmission Control Protocol (TCP) uses the MTU to determine the maximum size of each packet in any transmission.

Recently one of my relative had the following issue: 
Except from his mobile phone everyone is able to send/forward media using whatsapp. Later it was understood that it was a problem with android phones in his house. When it is limited to the house then the problem was understood to be with the SOHO router provided by the ISP.

After calling the ISP the following things were done by the tech support.

ping whatsapp.com -f -l 1292
Reply from 192.155.212.203: bytes=1292 time=316ms TTL=55
Reply from 192.155.212.203: bytes=1292 time=321ms TTL=55
Reply from 192.155.212.203: bytes=1292 time=316ms TTL=55
Reply from 192.155.212.203: bytes=1292 time=316ms TTL=55

ping whatsapp.com -f -l 1412
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

Then the MTU has been reduced from 1492 to 1432
Now all phones' whatsapp application is able to forward media.