How to add data to sql server using MS access front end, when user will not always have sql connection
This is the typical scenario for replication. You could set up a second copy of the SQLServer database on his machine (remember that SQLExpress has limitations on the size of the database - 10 GB - and does not have SQLAgent) using merge replication. 

The SQL Server instance you are using now could act as a publisher and the laptop could act as a subscriber. The Access frontend should point to the instance of SQL Server on the laptop instead of pointing to the publisher. When the user gest out of the wifi range, he would still be able to work using the local SQL Server instance. When he gets back to office, the subscriber could sync with the publisher.

Since SQLExpress does not have a task scheduler (no SQL Agent), you would have to use the Windows task scheduler for running the merge agent. In case you need very frequent sync, you could use the continuous switch in the merge agent.

Keep in mind that merge publishing a database is not totally painless: all published tables (articles) will need to have a uniqueidentifier UNIQUE column decorated with the rowguidcol flag. If you don't have such column, it will be added automatically, which will likely break all the insert statements that don't specify the column list. Moreover, all articles will get triggers for insert/update/delete operations and your workload might suffer a noticeable slowdown during write operations, depending on the shape of the workload itself.