Pionet Grid is not compatible with Meta Box yet so I think it will not work with the custom field that saved value in a custom table.
A product of the Pionet team compatible with Meta Box is Pionet Form which you can see here https://docs.metabox.io/compatibility/
I recommend contacting the Pionet team to consider creating an integration with Meta Box and extensions to support this case.
Following your link it clearly states that, "Piotnet Grid Builder (official support from Piotnet Grid Builder), is compatible with Meta Box. In fact, I bought Pionet Grid Builder on the recommendation by Meta Box given in my account settings. Now what..? I will go talk to Pionet. But how did Pionet Grid Builder make it to your list of compatible plugins?
Sorry about missing the info, "official support from Piotnet Grid Builder" that means they create the integration with Meta Box and support the integration. As you can see the integration with Elementor, the integration is officially supported by us.