You only see 500 Meg down with fiber? I just tested at 635M on copper coax to the house and Cat5e from the router (store bought Linksys). Disconnect the TV and monitor the speed- the router may have a log that shows this over time. If it doesn't have the same problem, check the settings in the TV and see if they're compatible with the router. I would also google search the problem- be as literal a possible. I would also search YouTube for network problems concerning that model of TV.
Can you post a photo of the router's labeling? Is it a modem/router, or two separate pieces?
Have you used the Quantum Fiber support page? It shows several model numbers.
I have worked with Sectrum (formerly Time Warner) and ATT systems and one thing I can say about both companies- the training and abilities of their installers is extremely inconsistent. Some delovered the wrong equipment, most don't know the answers to simple questions and tech support can be almost no help at all. Ask for a higher tier of support and if someone at the bottom askes if they can do anything, ask for Customer Retention (sometimes called 'Loyalty).