Allow VPN (PPTP) Connections for Mikrotik VPN Server

PPTP uses the GRE protocol, You have to allow ip PROTOCOL 47 (GRE), not TCP port.
TCP port 1723 is the control connection, while the actual tunnel is GRE (protocol 47).

Example:

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/ip firewall filter add action=accept chain=input disabled=no dst-port=1723 protocol=tcp add action=accept chain=input disabled=no protocol=gre