As per your query, your speed is irregular and your router SSID is different in your DLink router, what I am not sure here is whether the drop in connection is due to an intruder changing your SSID or something else, so I will discuss the troubleshooting techniques keeping both these things in mind. Follow the given instructions carefully:
Step 1: First reset your router since chances are that the intruder who has changed your SSID may have done some changes to your router settings, so press the "Reset" button at the back of your router for 20-30 seconds to reset your router to factory settings. This will eliminate and wipe out all the changes made to your Dlink router including the changed SSID. Also, make sure not to overdo this step as resetting your router for 5-7 times may cause permanent damages.
Step 2: Connect your DLink router to your computer using an ethernet cable, one end of the cable goes inside the WAN port while the other end goes inside your computer's ethernet port. Wait for few seconds to let the connection between the two get established.
Step 3: Open your web browser and type in your IP address in the given URL field and hit Enter. This will redirect you to your TP-Link router login portal. In case you do not know your IP address then you can simply google,"My IP address" or "Default IP address of DLink".
Step 4: Put "admin" in the username field and leave the Password field blank and hit Enter. Since you have reset your router the default user ID and password will work instead of the old one. This will redirect you to the main settings page of your router.
Step 5: Once on the main page, click on "Advanced Settings" > "Security", under the security section change the encryption of your device to "WPA2" as it is considered to be a government graded encryption type. Below the encryption, you will notice a box by the name of "Pass-key" this is your wifi password, change it to a secure and simple enough to remember and save the settings. A strong password must be a set of uppercase letters, lower case letters, and numerals.
Step 6: The above methods will help you secure TP-Link router but in case you want to make it impenetrable from outside source, you can use MAC Address filtering. Add the MAC address of the devices you want to grant access to your router under the MAC Filtering option and only these devices will be able to get access to your router.
Step 7: Since you mentioned that intruder changed the SSID for your router, go to the main page then to "Administration". From here you can change your router's SSID and login credentials just like the password for your router.
These methods will surely help you in making your DLink router safe and secure against any kind of intruder. Also, make sure to write down your password somewhere so that you can use it later