I suspect he was still an MP when he signed up. He might still be an MP now, at least until the by election. Even if suspended, which he might be, he is still an MP.
No, once he put in his official resignation letter he took the 'Chiltern Hundreds', which disqualifies him sitting as a MP, he's an ex-MP, and his constituency is unrepresented in the Commons until after the by-election.
As a constitutional convention, Members of Parliament (MPs) sitting in the House of Commons of the United Kingdom are not formally permitted to resign their seats.[1] To circumvent this prohibition, MPs who wish to step down are instead appointed to an "office of profit under the Crown"; by law, such an appointment disqualifies them from sitting in Parliament. For this purpose, a legal fiction has been maintained where two unpaid sinecures are considered to be offices of profit: Steward and Bailiff of the Chiltern Hundreds, and Steward and Bailiff of the Manor of Northstead.
Wiki
There is no real sanction for breaking the ministerial code for ex-MPs, apart from being 'told off', it's a waste of time and public money to investigate him over this, it simply doesn't matter, particularly as taking up positions with newspapers, radio & TV are always approved anyway, even for sitting MPs, but not ministers.
Hence the likes of David Lammy doing a show on LBC, Deluded Dorries' show on TalkTV, Rees-Mogg, Philip Davies and Esther Mcvey all doing shows on GB News. And, who can forget George Osborne breaking the ministerial code by becoming the editor of the Evening Standard, without seeking approval, and whilst still sitting as an MP, starting the job about a month before the GE, when he ceased to be an MP. Checking google, it doesn't appear he was even investigated for that, probably because it was pointless, as by the time it reported he would have been an ex-MP, and no sanction would be available.
Shadow minister calls for investigation into whether Osborne has broken rules governing former ministerial appointments
www.theguardian.com