In Manage 8.3.4, using MAF 8.7.34, I need to add 2 new sections to the WO Report option in the Technician application of Mobile (TECHMOBILE), both using custom objects  to which that must be added new records by using a custom dialog with custom lookups. The custom objects are relationated with the WORKORDER object in 1 to N relation how a detail information in the actuals tabs of the WOTRACK application. Also, new relationships & object structures for the custom objects was created on Manage

So far, I've managed to get the custom sections, custom dialogs and custom lookups with success, but I can't make tha those new records could be saved to the customs objects.  The app freezes and needs to be reloaded.

I did a copy of TECHMOBILE application, named TECHSLX and I have made several modifications, basically, in 3 files (app.xml, ReportWorkPageController.js and WorkOrderEditController.js) to create the new functionalities, such as new dialogs, lookups, datasources, replicating existing code in standard functionality similar to the one that is intended to be implemented.

Can anyone give me some clue about the possible problem?

There's a couple of things here I want to make sure you are aware of. Manage 8.3/MAS 8.7 are no longer supported and there have been a significant amount of changes to the framework and applications since that release. We're about to stop updates for 8.8. Customers should ideally be on 8.10 which is our first LTS release of MAS that will be supported for a minimum of 3 years since its release in March 2023. 

Also, we do not support any modifications to files outside of the app.xml and AppCustomizations.js. That's why in the MAF Configuration Utility we only display XML & the AppCustomizations.js file. If you're modifying the core JS files you're going to have a painful experience merging them back in for every patch. 

Regarding your issue, without understanding the exact requirements and how you've tried to implement it, it's very difficult to say what is going wrong. But before spending too much time on the customization I would focus on getting to a supported version and making sure that you're configuring the apps in a supported way.