LTE7490 MTU setting missing

Thank you, it's still a bit weird that MTU as low as 1277 is required for proper image loading on is.fi & hs.fi.

I tried "mturoute -t" command (free tool) to the is.fi image server (after putting client MTU back to the default 1500) where fragmentation happens (if MTU is over 1277), and it says 1464:

mturoute to 52.85.49.80, 30 hops max, variable sized packets

• ICMP Fragmentation is not permitted. *
• Speed optimization is enabled. *
• Maximum payload is 10000 bytes. *
  1 +- host: 192.168.2.1 max: 1500 bytes
  (steps between 2 and 14 omitted, either not reaching or 1464 bytes)
  15 +- host: 52.85.49.80 (image servers on is.fi) max: 1464 bytes

I guess for my PC→image server direction 1464 MTU works while for the opposite direction client MTU must be for some reason exceptionally low, 1277 ?

Attached are wireshark screenshots when MTU is set to 1277 and the images load properly:

So in conclusion:

Client MTU 1277: IPv4 packet total length 1277, TCP, no fragmentation, images load properly

Client MTU 1278-1500: IPv4 packet total length 1420, QUIC, fragmentation, images load very poorly

EDIT: OK! I've confirmed the issue is with my ISP: if MTU is over 1277, QUIC which uses UDP is enabled and my ISP doesn't seem to route it properly.

I tested 2 other ISPs (these are also mobile subscriptions) and with the default client (Windows network adapter) MTU of 1500, everything works great: images load properly, QUIC is enabled with no IPv4 fragmentation and IPv4 packet length is 1480:

DNA & Moi has this issue (reported, hope they'll get it fixed), Elisa & Telia are free of this problem.

Therefore your problem has a different cause.