If you get your IP blocked in Nextcloud, you can easily unblock it over cPanel > Terminal > occ client:
STEP 1:
Access Terminal over cPanel > Terminal and type: cd www
STEP 2
$ php occ security:bruteforce:reset <ip-adresse>
NOTE:
If you have an reverse proxy runnig you may want to add some special things to your Nextcloud config.php:
'trusted_proxies' => array('192.168.1.1'),
'forwarded_for_headers' => array('HTTP_X_FORWARDED_FOR'),
While 192.168.1.1 is the local IP address of your reverse proxy.