You need to connect to just just a single customer ? Then 'TeamViewer Remote Access' which costs approx 10$/month would be enough. Depending on the number of users, devices, concurrent connections the fee goes up from there.TeamViewer QuickSupport seems to perform well, but I am confused on what I need, as the "master" to purchase. Chatted back and forth with them, and they kept on promising to give pricing that is good for today only, but that never came.
And it is subscription only. It has become quite a bit more expensive than it used to be.
If you only need to get access to the customers desktop, then MS Teams may be an alternative. MS Teams is becoming dominant in the business world. Your customer is possibly already using it, and it may be easier to get acceptance from the customers IT department than any other solution. Each time the customer must initiate the session and must grant you the control of the desktop.
I have done a couple of remote training sessions with MS Teams.
There are a range of suppliers of canned solutions. We use E w o n.