I'm trying to setup the PPTP client on the router TP-Link TL-MR6400v5. It seems very easy according to the https://openwrt.org/docs/guide-user/services/vpn/pptp/client but it doesn't work. I know that the problem was described many times on the forum.
I noticed that even the client PPTP installed on my PC is not connecting to the VPN server if the PC is connected to the Internet via TL-MR6400. It seems that not only the PPTP client on the TL-MR6400 is wrong configured but something in global settings of the router blocks the traffic.
Not sure why you are having those issues, but it is worth mentioning that PPTP is considered unsuitable for the modern internet due to the triviality of cracking the encryption. It has been deprecated and removed from many modern operating systems, and it really should not be used unless there is a very specific reason, and only in situations where security is not needed at all.
I would recommend looking at wireguard as a replacement vpn technology.
This device is eol and unsupported. It would be a good idea to consider upgrading this device, too, as it is at least 3 years beyond its support window and may have many unpatched vulnerabilities that have been discovered in the past several years.
I made a test with version 21.02.1 and pptp client works fine from the router itself.
Both device (server and client) have public IPs and nat is not involved.
Do you see any errors in the log?
Sun Jun 19 18:55:08 2022 daemon.notice netifd: Interface 'pptp' is setting up now
Sun Jun 19 18:55:08 2022 daemon.info pppd: Plugin pptp.so loaded.
Sun Jun 19 18:55:09 2022 kern.info kernel: [1641365.600924] pptp-pptp: renamed from ppp0
Sun Jun 19 18:55:09 2022 daemon.info pppd: Renamed interface ppp0 to pptp-pptp
Sun Jun 19 18:55:09 2022 daemon.info pppd: Using interface pptp-pptp
Sun Jun 19 18:55:09 2022 daemon.notice pppd: Connect: pptp-pptp <--> pptp (37.157.x.x)
Sun Jun 19 18:55:10 2022 daemon.notice netifd: Network device 'pptp-pptp' link is up
Sun Jun 19 18:55:10 2022 daemon.notice netifd: Interface 'pptp' is now up