My first question is, are you able to change/modify the table on desktop?
For me currently it is a bit unclear what the goal of the table is. It looks like a table where the user can see the balance of an account (incoming/outgoing flow of money) (forgive me if I am wrong).
Yes I can change the table layout/structure
When I create an element in an application or a website, I always ask myself 'what question should be answered when the user looks or sees this element'. I will presume that this is indeed a table for the balance of incoming and outgoing cashflows.
As such I think it is best to highlight the difference between outgoing and incoming.
This example has focus on all information in 1 table.
Here you can click the 'outgoing' and it will dropdown to show the results.
Here you have 2 different tabs with the information
No I can not change the layout/structure
If you can not change the layout, it will be harder to make it user friendly. You can, as you mentioned put some text under the table. Another option could be something like this.
You can put an icon in the darker area that hints you can scroll.
(I am sorry for the poor mockups, just started using Balsamiq as a mockup tool to test out)