I doubt you are being hacked, but, there are several things you could do as a precaution.
First of all though, did the login you first saw look something like tr069@plusdls.net? That's the default autoconfiguration setup where you'd wait about 15 minutres for it to contact Plusnet's AutoConfig Server when it would then insert your username and password for you - so nothing to worry about there.
When you say your Broadband went down, do you know what the cause may have been - did you have a power dip for example, were you moving anything around and disturb any leads etc.? If the Modem and/or the Router loses power or connection to one another or the Line, even once the Modem has resynced and all the lights are green, you may have to wait 8 minutes (at present) for the stale PPPoE Internet session to timeout, before you can successfully try connecting again.
Should you ever wish to disconnect the Router for any reason, Log into it first, and go to the Internet box, and click on the Disconnect button. This drops the PPP(oE) Internet session to Plusnet. Wait about a minute, you can then power down the Router. Later when you power up, you log in and click Connect to establish a new session.
Don't disconnect or power down the Modem without Disconnecting in the Router and powering it down as above. After a minute you can power down the Modem but it's best to leave it off for an hour if you do that. And similarly don't reboot the Modem unless it's absolutely essential, powering down is a better option.
Back to the security precautions. Log in to the Member Centre to
change your password. Having done that whilst you are logged in, go to the
Plusnet Firewall and set it to Low, it's probably off at the moment unless you've been here and turned it on before.
Log out of the Member Centre and make sure you can Login to the Member Centre again with your new password. If for some reason you can't (or your old one) ring Support and explain what happened.
The new password will also apply to your Primary Plusnet email address, so you may need to change it in any email client you may have set up eg. Outlook (Express) or Mozilla Thunderbird etc.
Now Login to the Router (this password is unchanged and is as on the card that came with the Router) go to the Internet box and click Disconnect. Then delete your old password, enter the new one and click Connect.
It is possible that the minute you change the password at the member centre and log out, your Internet session may be automatically dropped, in which case come and login to the Router and change your password first of all.
By the way, if you want to Gateway hop you login to the Router, click Disconnect, wait about 30 seconds and the click Connect.
You can check
which Gateway you are on at any time.
HTH.
