Pay off the device plan and cancel the airtime without further expense to yourself.
The phone is then yours and you can take a sim only deal from a network that does give you a signal.
Remember to unlock your phone for free before you leave though.
http://community.o2.co.uk/t5/Other-Products-Services/Unlocking-an-O2-phone-to-use-a-different-SIM-ca...
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)