You can use the online flag on your router outlet with an ngIf
<router-outlet *ngIf="online$ | async else offline"></router-outlet>
<ng-template #offline>
content here displays when not online
</ng-template>
or
<router-outlet *ngIf="online$ | async"></router-outlet>
<ng-container *ngIf="offline$ | async">
content here displays when offline
</ng-container>