SPF records are to declare which servers may send email from the domain.
You say your email accounts are with 123reg, so presumably their servers will be a source of emails from your domain, so you will need to reference 123reg servers in your SPF record.
When you say
the domain is hosted at IONOS (1&1)
what is hosted? If something there may send emails "from" your domain, you might also need to include IONOS's SPF record.
According to this:
https://www.123-reg.co.uk/support/domains/how-do-i-add-an-spf-record-to-my-domain-name/
The record for 123reg's sending servers record will depend on the "generation" of your mailboxes.
This:
https://www.ionos.co.uk/help/domain...ds/using-ionos-spf-to-improve-email-delivery/
gives the IONOS record at the bottom of the page.
You only need to include the provider(s) that actually sends emails from your domain, but if it might be both you'll need to create a combined record with both of the include: statements (there's a bunch of SPF record creators online if you're not sure how). You should only create one record, but it needs to reference all the possible sending servers.
Once you've confirmed all the potential legitimate servers that may send emails from your domain you need to turn to whichever provider runs your DNS servers to add the record. Usually that will be your registrar (so 123reg), unless you've made other arangements.
The 123reg page says you need to move your DNS servers to them to add an SPF record. That doesn't make sense - if you have your DNS elsewhere, then just add the required record using the DNS host's management interface.
