@kim_james 

I'd check the RJ45 cable from the router to the PC first, also before you buy a new one ensure the plug is in the router properly or select another port on the back of the router and also in the PC as they don't always make a full connection or can get tugged out a bit when things get moved around, best test from the PC is to physically push and hold the cable gently in the ethernet port on the PC.

I assume wifi works fine but wired doesn't?

@Hannibal_Lack had done that, wifi is fine. Three PC's on ethernet suffer the same fate.

The only remedy I have found that seem to work is to reboot the modem before I use the PC.

@kim_james 

Yeah that's what makes me suspect your RJ45 cable, if you don't have another simply disconnect it from both ends and swap it round so the ends are now at the other ends of each other to where they were, probably won't help if the cable is bad though but 2 mins of time so not a big issue to check out,

Also another long shot, do you use the whitelist/blacklist function in security settings? if so turn it off and test.

@kim_james 

Hi, it is suggested to test the LAN connection with different cables, and you can change the modem's DNS to 8.8.8.8/8.8.4.4 to give it a go.

When the PC loses the internet, does it still able to obtain an IP address from the modem? Please refer to the instruction below to check the IP address. How do you recover the connection?

https://www.tp-link.com/support/faq/838/

What's the light status of the modem? When did you buy the modem and when did the issue happen? Does it work fine before?

Do you mean if you reboot the modem before using the PC, you will able to get a stable connection?

@kim_james

A few more things came to mind later, the VR600 can be used under 3 different purposes so the one you need to use is the DSL setting, not 3G or Wireless only modes, this is found in the Operation tab in the routers settings pages, ensure you select the DSL one,

Also what Stella has said can be checked easily by viewing the basic tab in the router and selecting Wired Clients, if your PC is showing there then the cable is fine and the fault is elsewhere, deffo get another RJ45 cable and if that and the setting for DSL doesn't sort it it's possible that the unit is failing,

Also I would try this as well before giving up I'd do a factory reset and after setting up go in the System Tools -> Admin page enable local management and use just one MAC address for a trustworthy connected device, this will rule out someone possibly in the house accessing the router page and changing settings, as Stella said use a different DNS, she's suggested Google @ 8.8.8.8 backed up by 8.8.4.4 but Cloudflare is equally as good if not better on 1.1.1.1 backed up by 1.0.0.1

https://www.theverge.com/2018/4/1/17185732/cloudflare-dns-service-1-1-1-1\ 

If it isn't a bad cable I can only think it's configured wrong or failing on the LAN side, hth.

@Stella.Q 

DNS has been on Google servers

PC still has IP when it loses the internet

What's the light status of the modem - all lights are on 

When did you buy the modem - Oct 2017

and when did the issue happen - since I bought this modem

Does it work fine before - nope