Sync error with linux client 2.6.x with debian 10


i have problem syncing with nextcloud client runnning on debian buster (4.19.0-6-amd64) since client version 2.6 with my internet service provider, this is quite particular:
i can connect with 2.6.x client when using wifi via a smartphone, but it refuses when using my isp connection (which name is Free)
i can connect with browser when using wifi via a smartphone or my isp provider connection,
no problem too when i m near the server on same local network

i checked the mapping and firewall, nothing seems to be wrong since it works with firefox browser

does someone have same problem?

ipv6 (not properly configured)?

i think iā€™m obviously a beginner with ipv6, do you know where i can find some documentation?

And also the question if you are using it within your network. Is it just your smartphone or other devices on your wifi as well? Other devices are a bit easier to debug.