Radio signals travel further at night so could be picking up a distant transmitter.
The status page will tell you if 4g is coming anytime soon
http://status.o2.co.uk/
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion.
This is not Customer Service so we are unable to help with account specific issues.Currently using:
Pixel 7a (O2 & Lyca), One Plus 6 (Sfr), iPhone 12 Pro Max (Vodafone)