Solução A. Abra a opção de passagem PPTP no seu roteador (com o encaminhamento de porta 1723).
Solução B. Defina DMZ para a caixa do Windows (sem encaminhamento de porta 1723).
(Cation: Desligue o firewall na caixa do Windows durante o teste.)
Se nenhum dos itens acima funcionar, você precisará substituir o roteador por outro que suporte a passagem de VPN.
PS: PPTP não é seguro (google para descobrir o porquê). Se você quiser segurança, use outro protocolo VPN (como L2TP sobre IPSec).
Update: (20161021 14:31 UTC)
@bph Sorry for not thinking this through.
PPTP Passthrough is for PPTP clients. If all clients are behind the same router, the router need to support PPTP Passthrough for multiple connections.
Please check this link for PPTP Passthrough in WAG320N: http://setuprouter.com/router/linksys/wag320n/vpn-13525-large.htm
In your case, the PPTP server is behind the router, so you have to configure DMZ in the router to forward GRE packets to the PPTP server.
Since you mentioned you'd set the DMZ in the N150 router, did your Android phones use N150's WiFi network or mobile data network? If the Android phones used N150's WiFi network, the failure could be caused by N150 not supporting PPTP Passthrough. But if the Android phones used mobile data network, this will be complicated and I need further info (such as Windows box system version and PPTP server service version) to troubleshoot this problem.
PS: Check this link for further understanding (PPTP Traffic Analysis by The Cable Guy - January 2003): https://technet.microsoft.com/library/bb877963@ fixer1234 @ Mokubai ♦ Obrigado por me lembrar. Eu devo ter criado a conta adicional por engano. Vou enviar a solicitação de contas de mesclagem em breve.