nsis - how to set "Next" button as default button

Question

I have two custom pages.

In the first custom page, the default button (triggered on <Enter>) is the Next button.

In the second custom page, I set the default button to a different button:

SendMessage $HWNDPARENT ${DM_SETDEFID} 1221 0

But when I click Back going back to the first custom page, the default button there is no longer Next (i.e. <Enter> no longer triggers Next).

How do I set the default button in the first custom page back to Next button?

I tried the following but it doesn't work:

GetDlgItem $1 "$HWNDPARENT" 1
SendMessage $HWNDPARENT ${DM_SETDEFID} $1 0

EDIT:

Code and file structure is actually complicated, but I'll try to simplfiy:

Dialog1.nsh

Function fnc_Dialog1_Create
    ; create dialog controls here
    Call SetupDialog1
FunctionEnd

Function SetupDialog1
    ; create event handlers here
    nsDialogs::Show
FunctionEnd

Dialog2.nsh

Function fnc_Dialog2_Create
    ; create dialog controls here
    ${NSD_CreateButton} 217.87u 126.15u 78.33u 14.15u "$(TRANSLATED_DLG_BUTTON_TESTCONNECTION)"
    Call SetupDialog2
FunctionEnd

Function SetupDialog2
    ; create event handlers here
    ; set Default Button (prevent "Install" on <Enter> by setting default button to a different button
    SendMessage $HWNDPARENT ${DM_SETDEFID} 1221 0
    nsDialogs::Show
FunctionEnd