Router problem?

I'll get the interrogation started.   More information will help.

What model router? Has it worked before with this equipment or is something new in the set up?

When you use the router, are you connecting the WAN port of the router to the modem, and the computer to a LAN port? If so, have you checked the cables by using both to just connect the PC?

It could be that the router DHCP is turned off so is not assigning an address to the PC, or perhaps you need to clone the PC MAC address to the router depending on your ISP requirements. I would start by opening the router configuration pages by connecting to the PC and typing 192.168.1.1 in the browser window, then the user ID and password (from your manual if not changed).

what do you mean by model router?
it works fine before, and i didn't do any new set up

may be i was a little confused between router and switch , or they are same thing?
the first photo was when my network conf like this : my_pc - modem (1st conf)
the second one : my_pc - switch - modem (2nd conf)

by using 1st conf, i could access 192.168.1.1 via my browser but i couldnt when i use the second conf.

What are the brand and model number of both your modem and you switch, so I can figure out what you have to explain how to hook it together?

Switch : TP-LINK Model : TL-SF1008D
Modem : TP-LINK Model TD 8817

Okay, that helps. At least now I understand what you have -- a modem with one LAN port and NAT capability and an 8 port switch. Are the power and port lights working on the switch when you plug in a cable?

Unless a cable is bad (you can test each by connecting them from computer to modem directly) then the problem sounds like the switch -- try some other port combinations as occasionally a port will die. Otherwise try a new switch if you can borrow one or buy one where you can return it easily.

thank you very much
i thought that was the problem too, it has been used this modem and switch for more than 3 years ... 24/7
my modem not working so well too this day, sometime it works well, some time not
is it okay just to buy new modem and switch without contact my ISP?

Yes, as long as the models you get are compliant with your ISP, many of them have pages that list models that they recommend, otherwise any ADSL2+ modem and any router will work.