Moving the modem to the VFC 0421 splitter should improve the signal strength and quality, improving internet connection speed and stability as a result. You will need an F/F cable (red connectors), which should have been part of the modem's installation kit.
The VFC 0421 should ideally be replaced with an ESX-02, which is available from Ziggo free of charge.
However, since we did not manage to find the AOP and it appears to be located outside of your property, the other part of the installation (in the inaccessible area) should also be verified.