Can we get DSL light without ISP's username password in model in PPPoE connection?

I am stuck into a problem, my broadband is disconnecting every now and then, DSL light of my TP-Link TD-W8968 turned off every now and then and I am unable to surf. Then I have to wait for few mins. to get the DSL light on, in my modem. I thought this is BSNL's problem I registered a complaint, first they told me that is it because of your internal wiring, they checked the SNR at the first connection of my house and then checked the SNR at the modem's end, they are saying that we are getting more than 20 (I am unaware of its unit) at the first point and around 6 at the modem's end. So spent few thousand rupees and get my internal wiring change, earlier Finolex cable was installed now I have installed the cable which BSNL people have given me. Now the problem is still there, DSL light is going off again and again. Now they are saying that my modem is faulty, they have given their modem ( not sure of the company but SSID is Asus) to use for some time. I tried to ping google.co.in website reply is breaking but DSL light is still on. When reply is breaking in pinging the connection is also breaking. But they are arguing with me that DSL light should be on even without ISP's ID and password, and your modem is not showing but ours is showing means your modem is faulty. When they come to my house for rectifying my problem I am usually in the office and no one knows about internet and all, in my house other than me. So they are keep bluffing me.

Please let me know can we get DSL light even without ISP ID and password? As far as I know we can't get DSL light without ISP's ID and password in Modem especially in PPPoE connection. Please help me guys.

Yes, DSL light can be ON even if you haven't entered the username and password given by ISP if the default configuration in the modem matches BSNL's DSL config (VCI/VPI, modulation type etc). Post your line stats here, which shows line rate, SNR margin and attenuation.