IPv6 and MTU problems

I have an LTE modem in bridging mode as my uplink. Lately, there have been weird problems and what I could find is that it's caused by some of ipv6 traffic just getting stopped.

As a minimal test, I can, using either my openwrt box or a laptop connected directly to the modem:

  1. ping -6 ping.funet.fi -c 1 -s 1232 # success
  2. ping -6 ping.funet.fi -c 1 -s 1233 # FAIL
  3. ping -6 ping.funet.fi -c 1 -s 1452 # FAIL
  4. ping -6 ping.funet.fi -c 1 -s 1453 # success

So, I'm suspecting MTU brokenness. If I understand that correctly, traffic goes through (and it does, mostly) if small or if fragmented but what fails is that fragmentation starts at wrong size. There is one MTU setting on the modem which I suspect will do nothing.

At the moment, I've turned off RA relaying to stop the brokenness (and ipv6 traffic out) in my network.

This is probably an ISP problem, but if they won't fix it, is there any way to go around the problem on the client side?

IPv6 MTU sizes for common LAN and WAN technologies -

First of all, testing with just one packet doesn't prove much.
Second, if MTU was broken on the ISP side, they would have problems with all of their customers.
Third, you need to upload the OpenWrt configuration to verify whether it is correct.

Please run the following commands (copy-paste the whole block) and paste the output here, using the "Preformatted text </> " button:
Remember to redact passwords, MAC addresses and any public IP addresses you may have

ubus call system board; \
uci export network; \
uci export dhcp; uci export firewall; \
head -n -0 /etc/firewall.user; \
ip -6 addr ; ip -6 ro li tab all ; ip -6 ru; \
ls -l  /etc/resolv.* /tmp/resolv.* /tmp/resolv.*/* ; head -n -0 /etc/resolv.* /tmp/resolv.* /tmp/resolv.*/*

Fourth, any problems that the modem of the ISP might have are outside of the scope of this forum.