index.php in permalink

Due to the vagaries of my WP installation, I used to have permalinks in the following format:
OLD FORMAT: https://mysite.org/index.php/%year%/%monthnum%/%day%/%postname%/

at the time, “index.php” was necessary to make stuff work. My WordPress .htaccess file was standard.

Recently, my hosting company migrated me to another server. Now the old format format does dot work. A different permalink format, without the “index.php” does work:
NEW FORMAT:
https://mysite.org/%year%/%month%/%daynum%/%postname%/

Since there are many legacy links in my blog and on third party sites in the old format, I would like to maintain the old format. I tried inserting redirect and rewrite rules into the .htaccess file, but to no avail.

Help, please?