I’m not a big fan of Ionic’s grid, so I can’t speak directly to it. I think you need to specify some sort of height on your scrollable area; take a look at the following and see if you can adapt it to your situation:
<ion-app>
<ion-content scroll-y="false">
<div>i am fixed above</div>
<div class="grid">
<div class="cell" *ngFor="let fruit of fruits">
<div>{{fruit}}</div>
</div>
</div>
<div>i am fixed below</div>
</ion-content>
</ion-app>