no-one can magically pull answers out of the air. 405 errors are NOT commonly seen with wordpress unless there is a wonky server set up.
If you want me to help to help, then I need you to:
Do this:
copy the following into a file :
<?php phpinfo(); ?>
name the file whoomeg.php
Upload whoomeg.php to your web space so that I can look at it in my browser.
Come back here after you have uploaded, letting me know the URL to the file, so that I dont have to hunt for it.
Then, do this:
Go download your error logs (NOT your access logs), and LOOK at them to see what the error says.
https://www.one.com/en_US/support/guide/advanced/logfiles
^ There are directions for your host (I think).
I honestly dont know if 1&1 does error logs. They may not, but you dont know until you look.