I have had this problem with two TL-MR6400 and now an Archer MR400. It relates to setting up a VPN from and external PC onto the 4G network of the MR400.
No problems connecting up via DDNS using either PPTP (used for testing only) or via OpenVPN. Once connected I can ping the gateway of the MR400 (in this case 192.168.3.1) but cannot ping anything else on the 192.168.3.XXX network (two other devices on there). If I use advanced IP scanner I can see other devices but cannot connect to them.
Any ideas would be very welcome.
The odd thing is we have tested three TL-MR6400's and one of the three worked fine. All running the same firmware. We need two to work though.