(I am currently using ExpressVPN, but regardless what name it is I think the issue that I have will still apply)

When use a VPN:
1) there any settings that has to be changed to the ISP's router, connected to the PC, in order for (my) ISP not to see my activity (i.e. downloading /uploading, and how long) ?

2) or just simply chose a location on (in this case) Express VPN location - "smart location" or any other, but not yours ! (This being only for the purpose of copyrights, basically ISP not to know what someone is downloading, but still seeing how big of a file how long /in time ? Thus changing anything in your router's settings for that purpose, wont help !? )

(Notes:
- Months ago when I was new to torrenting, I left my Torrent Client (utorrent) open 24/7, thinking that since I use a VPN the ISP wont be able to see my activity. Later I received a letter, saying to check my connection, in case someone else is uses my IP address. If it continues, they will disable my account, even tow, is "unlimited" download when I signed-up, or so I thought.
- that is why (in my thinking) I asked above, if any changes in router's "setting" provided by ISP provider ??? )
just confused

Please help

I'm a little unsure on your exact question, but I can say that in terms of a 'copyright strike' relevant email (usually from a company to your ISP, which then your ISP will notify you).. this is not something you need to worry about using a VPN. Try a site like ipleak.net and open the magnet detection in your torrent client (with VPN on) and check that ipleak shows the IP of your VPN and the VPN DNS server(s). It's a great way to test.

In reference to your ISP contacting you previously, I would think this is in relation to a noticed amount of traffic on your account. In the fine print of many ISPs the 'unlimited' plans are still governed by a fair use policy, where through inspection of your accounts usage they may have noted significant upload and/or download quantities, hence they have contacted you. Did they specifically mention it was from torrents? As far as I know they should not be able to inspect the traffic if it was masked by VPN, they should only see the amount of data on a throughput. My country's ISPs are commonly known to warn customers about excessive usage when on an 'unlimited' 4G mobile network plan. But it does vary from country and ISP.

I hope this helps.