I changed my broadband from excitel to RailTel. They don't bind mac. Now I am getting full speed with xx530v. 
 

Also there is no issue with using blue cable with green port. So its confirmed. Issue is with tplink firmware. 

  @shu006 Oh that's nice. 
I had a talk with TP-Link's engineers today and after changing some settings (just changed DNS Servers 🙄) and running multiple speed tests he concluded that the ISP is throttling the speed due to 3rd party ONT. It could be true, but I still not convinced.
I'd like to run some tests with another 3rd party ONT from V-SOL so I have concrete proof that its a TP-Link firmware issue, if it really is that. But I'd have to shell out another ₹3000 for that ONT.
You could help me out a bit, so maybe I wouldn't have to buy it. If RailTel doesn't bind MAC at all you can try running 2 speed tests-

1. One with MAC Cloning Disabled
2. One with MAC Cloning Enabled (put any Random MAC address). 

If with MAC Cloning download speeds are again being limited to 50Mbps and without MAC Cloning speeds are back to normal, that would be great proof that it's really a firmware issue.
I could guide you through it if you're up for it and need help. We can gather Screenshots and Screen Recordings as proof.
It would be of great help for all of us having this issue and TP-Link would at least look into it more seriously.

Also there is no issue with using blue cable with green port. 

Yeah, there won't be any issue per say. It'll work perfectly normal. You would just get lower RX power, but won't affect your internet speeds.
For example, I have the Blue connector by default (SC/UPC), directly connecting it to a Blue port on the ISP router I get -19.6dBm. Putting the Blue connector in the Green Port on the XX530v i get -25.7dBm. Using a coupler and patch cord to adapt the Blue connector to a Green connector (SC/UPC → SC/APC) and connecting it to the XX530v I get -21.5dBm.

Looking forward for your reply.

@shu006  Hey, I sent you a direct message can you check it and get back to me.

So I tried with few ISPs. 
ISP 1 - Asianet -  PPPoE - 200 mbps  plan. I tried with the cloned MAC, but the download speed was throttled to 50M. The upload speed shooted upto 200M. Since I knew the LCO, i asked him to release the old MAC and tried with the XX530 MAC. There was no throttling, i was getting the same speeds that i paid for.

ISP 2, Asianet - IPoE - 200 mbps plan. There was no problem after cloning the MAC. I was getting the assigned speeds i. e. 200M.

ISP 3, KeralaVision - PPPoE - 200 mbps plan. Had to clone the MAC address of the original NETLINK router, then the download speeds capped to 50M. There was no problem with the upload speeds. The LCO didn't even bother to release the mac address. So i stopped the testing there.

Hence I got a conclusion that if the connection is pppoe and if we need to use the mac address other than tp link's (cloning), the download speeds gets throttled to 50M.

  @grad1e Hey, that's some valuable information you gathered right there. Thanks!

I also ran some tests of my own to rule out that its my ISP that is throttling the speeds due to it detecting 3rd party device on network. What I found out makes it more evident that its something with the firmware.

What I did- 

1. Put the XX530v in Bridge Mode and disabled the PPPoE interface on the router.
2. Cloned the MAC address of the ISP provided ONT onto the Ethernet adapter of my computer. This is the same MAC that I'm cloning on the XX530v as well.
3. Setup a PPPoE Dial Up profile on the computer and connected to it. Authentication was successful and I had internet.
4. Ran the same 3 speed tests as the ones ran by the TP-Link Engineer so he when I tell him he at least he believes.
5. In all the speed tests I got my full designated speeds of 100 Mbps Up/Down.

This hardens the fact that the firmware is faulty. 

Tomorrow I'll contact the engineer again and inform of all the findings we have gathered.

@alb3530 @grad1e @shu006 
I have complied a formal report that I'll be forwarding to TP-Link. I hope you all don't have any objections as I have mentioned you all as contributors and quoted your messages
Attaching it below-

  @Yash_Kanawjia I have the VSOL 2802RH, HSGQ SFP XPON ONU, Archer C64 and Archer C6. I can use that to verify that the firmware of XX530 is surely the cause. But i might need a few days to test it out since I am currently at a different place right now

  @grad1e That would be great! This would surely rule out any throttling done by the ISP and put all focus on the XX530v's faulty firmware. You may do the testing at your leisure, I don't think there's any reason to rush getting the results from the other ONT/ONUs as I believe the evidence we have collected up till now should be plenty for them to take it as a router-related issue, if they don't remain ignorant and actually listen.
Just talked to the engineer 15 mins ago, I told him verbally what we discovered and he said 'so throttling is when MAC Cloning is enabled. Okay, I'll check from my side' (translated from Hindi). Also forwarded him the Report made and screen recording of me doing PPPoE + MAC Cloning from Windows and getting full speeds in speed tests.
Will update you once he gets back to me. I hope they don't brush it off and call back to give updates on the issue.

Also you're not the guy from the YouTube channel Geeky Twins, right? I mean, I was amazed when I read you had the VSOL 2802RH, HSGQ SFP XPON ONU on hand. He has the same ones and uploaded videos about it. Plus, you already have the XX530v like him, so three coincidences in a row left me stunned! 😂