If you are willing to upload files larger than 511 MB, please check this guide:

https://docs.nextcloud.com/server/9/admin_manual/configuration_files/big_file_upload_configuration.html#configuring-your-web-server

If even your 2MB files fail, check all size limits in the following settings files:

.htaccess

root@f3d81bd814d4:/# grep upload /var/www/html/.htaccess 
php_value upload_max_filesize 511M 
php_value upload_max_filesize 511M

Nginx conf

If it is running, you should have /etc/nginx/nginx.conf file, where client_max_body_size needs to be configured to, at least, 512M

root@f3d81bd814d4:/# find . | grep nginx| xargs grep size | grep -v '#' 
./usr/src/nextcloud/core/doc/admin/_sources/installation/nginx.rst.txt: client_max_body_size 512M; 
./usr/src/nextcloud/core/doc/admin/_sources/installation/nginx.rst.txt: client_max_body_size 512M; 
./usr/src/nextcloud/core/doc/admin/installation/nginx.html: <span class="kn">client_max_body_size</span> <span class="s">512M</span><span class="p">;</span> 
./usr/src/nextcloud/core/doc/admin/installation/nginx.html: <span class="kn">client_max_body_size</span> <span class="s">512M</span><span class="p">;</span> 
./var/www/html/core/doc/admin/_sources/installation/nginx.rst.txt: client_max_body_size 512M; 
./var/www/html/core/doc/admin/_sources/installation/nginx.rst.txt: client_max_body_size 512M; 
./var/www/html/core/doc/admin/installation/nginx.html: <span class="kn">client_max_body_size</span> <span class="s">512M</span><span class="p">;</span> 
./var/www/html/core/doc/admin/installation/nginx.html: <span class="kn">client_max_body_size</span> <span class="s">512M</span><span class="p">;</span>

Nextcloud configuration

Settings > Administration > Settings > Basic settings

File handling

Maximum upload size

With PHP-FPM it might take 5 minutes for changes to be applied.