Need ONU suggestion for BSNL FTTH

I recently changed my BSNL plan to 999/m Fiber Premium (the change hasn't gone through though for some reasons). I was using a Netlink 2.4 GHz router but to take advantage of 200mbps I will have to switch to dual band router. I wanted to know what would be the best way to go about this. My LCO suggested getting any 5 GHz router and connecting it to the existing router via ethernet to get the full speed. Will that be the best way to go or should I ask them to replace my current ONU with a 5 GHz one (I bought the router when getting the connection so it's not on rent). Also, I was using TP-Link WiFi extender (This- TP-Link TL-WA855RE 300 Mbps WiFi Range Extender - TP-Link : Flipkart.com) which is 2.4 GHz up to 300mbps. Will I need to change that as well or will it be able to connect to a 5 GHz router and deliver full speeds.

You could procure a TP-Link Archer C6, and use it with your existing ONT. It's the most VFM option right now.

@varkey Okay will check it out. Thank you