Odd!
I think that GMX is filtering my email normally, although I do notice that an email from Patisserie Valerie has gone into my Inbox today, rather than straight into 'Spam', where such emails usually go. Other email destined for 'Spam' seems to be ending up in the right place though.
I assume that you're using the iPad's own Mail app. If so, open your web browser instead, go to
https://www.gmx.co.uk and log in there to see if spam emails are being directed to your 'Spam' folder on GMX's own server. If they are, the problem would appear to be with the Mail app on your iPad.
Assuming then that the problem is with the Mail app, try deleting your account and then recreating it. (You won't lose any existing mail). To delete the account, go to Settings, tap Accounts and then tap the name of your GMX account. On the details page for the account, tap Delete Account and then confirm the action by tapping Delete. To recreate the account go to Settings > Accounts again and tap on Add Account.
Alternatively, if you're currently using the iPad's own email app, simply switch to using the dedicated GMX one. (I use the Android equivalent on my phone and I find it very user friendly):
https://apps.apple.com/gb/app/gmx-mail-cloud/id417352269