Ah, I see.
You are trying to display the new column in on the front-end and not interact with it on the back-end.
You'll need to update the toApiArray function in the client module to also return the new column as the back-end doesn't make all of the columns available to the database from the get-go.
Also, I'd generally be hesitant to modify default tables in FOSSBilling, especially by changing what you are trying to use already existing columns for & redefining it's structure. Although it's unlikely to get changed at any point in the future, if we push an update that modifies that column / table it may break what you've done.
And in a similar point, I'd generally avoid modifying the source files in FOSSBilling as those changes will be overridden when you update. For front-end work you can use a custom theme and that'll avoid that issue, but for other items the best route to go would be to create custom modules so that you aren't creating modifications that will be reverted after each update