Can you just remove the splitter AND your VHF radio to see you will get better receiving range?

The following link should be a help to you.

https://www.quark-elec.com/doc/data/HOW_TO_MAXIMISE_YOUR_AIS_RECEPTION_p.pdf

In most cases. the receive range issue can be down to two points: Power and antenna.

A clean power is critical to the RF device. Try a clean power bank(fully charged) to see if it helps?

Check the connection of the antenna,especially the connectors.

Try not to use adapter if possible.