Buying additional national insurance years to qualify for a full state pension

I took early retirement in 2007 when I had enough NI contributions (30) for a full state pension, then the "goalposts" were moved in 2015 & now I am short of 5 years of contributions. I reached state pension age in 2021 & enquired about adding to my contributions. Unfortunately I was diagnosed with cancer at the same time, so I shelved the idea. Now I understand there is a deadline, but can I buy contributions for the years, say for example 2016-2020 in the future, & leave 2007-2015 blank, thus attaining 35 years of NI contributions or is their also an urgency to do this before the deadline.

Hi,
The deadline to pay tax years 2006 to 2007 - 2018 to 2019 is 5 April 2025. 
You can still pay tax years 2019 to 2020 onwards after 5 April 2025, however the payment amount needed to fill the year will change every tax year.
If you have not done already you must first speak to the DWP (Future Pension Centre) only they can specifically tell you which years and how many years you need to pay for.
We understand that due to the high demand on telephone lines that some customers are struggling to get in contact with DWP.
DWP have launched an online form so that any if you are unable to speak to DWP before the deadline you can register their interest in paying voluntary NICS and request a call back.  
If you submit a request by the deadline, you will be able to pay voluntary NICs after the deadline has passed at the rates that were applicable in the 2023 to 2024 tax year.
Thank you