Huawei B528 with another supplier?

Can one use the Huawei B528 also for example with the O2 net?

This is called simlock! It forces you to stay with the provider who presumably "sponsored" the smartphone within your contract term.

If the minimum contract period has expired, you can remove the simlock, so that the smartphone also works with other mobile service providers.

Google / search for simlock (at the "sponsor" / mobile operator of the smartphone) or read this:

https://www.toptarif.de/handy/wissen/simlock-entfernen/

The device is not a mobile phone

Sorry, … It's an LTE router.

But I think that works with any SIM card that has a "LTE data plan".

But check in advance if you have any good LTE coverage in your planned area of operation.