iPhone mail not showing all mails

My perfectly and updated iPhone X will not show all my mails in Apple Mail - can't figure out what to do.

My phone is updated with the last update (this week). Phone is rebooted several times and I have several e-mail-accounts. My main and standard account for my own domain is an IMAP. I can not set anything regarding synchronizing in this account.

On my iPad (updated as well) I can see all my mails - and in webmail I can see them as well. But it seems like they got hidden (some how) on my phone. I also checked the filter (dashes in lower left corner) but nothing helped.

The only thing I have not done - and would like to avoid - is deleting the account and set it up again.

Any other suggestions?