No Mobile Coverage draining iPhone battery

We have recently been on a Canary Island cruise over Christmas and New Year. Having uploaded the latest IOS before we went. We noticed the battery on my wife's iPhone was draining at an alarming rate with no idea why. There was paid WiFi on board which we did not use so a lot of the time, no mobile connection. Though my wife had an e sim which was used on and off.

Looking at the battery info, the phone seems to be permanently looking for a network. As "no mobile coverage" was draining 20% to 40% of the battery during the day. My own phone used some battery on both 25 and 26 December.

Having Googled, we found the only way to stop this was to put the phones onto airplane mode.

We have never had this issue with our phones abroad before, this seems to be something that has only happened since the latest IOS upgrade.

Has anyone else encountered this issue recently? Thanks.