my imessages gets deleted by themselves continueisly

this is happening by some months more than 6 months, i contacted apple support several times and none of the tips were helpful because my problem did not end up being fixed.

i tried restarting my phone, update , turn on keep messages forever and all those options but it doesnt seem to work, my messages doesnt even apper as an app in icloud web.

the messages i lost are so important to me there been some serious personal stuff.