@Patrick_Kalka 

https://community.tp-link.com/en/home/forum/topic/168500

The post is from 2019, but I think the situation hasn't changed. No VPN client options for home or SoHo routers.

@woozle thanks for your answer. I'm afraid I don't get the point.

I bought the mr3020 because it has an Vpn client. 

@Patrick_Kalka 

I see the MR3020 supports what is commonly being refered to as "Site-to-Site VPN". It can be used to connect two networks via a secure VPN connection.

You'd have to check if your Synology supports this as well.

@woozle I also posted at synology, but no response yet. 

Assuming my mobile knows what it's doing, it is not possible to open vpn without user credentials.

That is what mr3020 is missing. 

Or should there be a way without?

@Patrick_Kalka 

There are the Client -> Server VPN setups, usually being used to allow a user to securely connect his/her device (mostly computer or smartphone) from a remote location to a network, like a home network or company network. The client device needs to support the same VPN protocol that the server is using. For example, Android phones have client support for common protocols like PPTP, L2TP/IPSec PSK built in, but for OpenVPN an additional app needs to be installed. 

Then there are site-to-site VPN setups, being used to connect two (or more) networks together, for example the branch offices of a company at different locations or two home networks for people who have a secondary residence.

Unless I am missing something, the TL-MR3020 only supports the so-called site-to-site VPN. So you would need a device on the other side that also supports site-to-site VPN and the same protocol to establish a VPN connection.

Edit: If you want to see how the configuration GUI looks like on a TP-Link router that can be configured either as VPN Server or as VPN Client, then have a look at this emulator here:

https://emulator.tp-link.com/TL-R600VPN_UN_4.0_Emulator/index.html