Welcome to the community! Would you mind letting us know the following so that we could better understand your situation:
Could you kindly let us know the version of the KoBoCollect android app are you using for these two devices? If you are not using the latest version i.e. v1.25.1 would you mind trying it out and see if you still see the error message?
Do you see the same error message in other devices as well?
Welcome to the community! Would you mind providing us a detailed outline so that we could help solve out this issue. Maybe would you mind starting by sharing a screenshot of the error message you are seeing.
This is normally an issue with either the browser they are using. Can you ask them to use chrome and use an updated browser as hinted by @Kal_Lam on a previous post which can be found here
Request them to open a browser and see if they are able to connect to an internet. Sometimes an internet connection also creates connection errors.
If they are able to connect to an internet but not able to open the survey form request them to upgrade their browser to the latest possible version. Avoid using Internet Explorer . Try using Chrome or Mozilla. Sometimes the browser also causes this sort of issues.
Request them to use the exact characters that has been shared with them. Sometimes a typo error could also lead to this issue. FYI, while trying the link (you provided) in my tablet i too had a typo error and could not access. But then i corrected the typo and i could open the same in my tablet as well. .