For best results waterslide decals need to be applied to a gloss surface. Below is my approach which may assist.
If you've retained the backing sheet for the large logo I would remove the decal from the tender and slide it back onto the backing sheet to reuse. If not slide it onto greaseproof or wax paper if you want to reuse it.
Apply a thin gloss varnish to the tender sides. If it were me I would also be removing most of the carrier film from between the white double arrow.
Once the gloss varnish has dried slide the decal back onto the tender side. When applying decals I use Microset and Microsol from Microscale Decals (widely available in the UK). Others are available.
Microset acts as a wetting agent by breaking the water surface tension. It is a very mild acetic acid (white vinegar) and helps the decal lay flat.
Once I have the decals in position I brush them with Microsol being careful not to move them in the process and leave to dry. As it says on the bottle it softens the decal and draws it over irregular surfaces e.g. rivets. Once dry I apply a coat of varnish.
I have used both solutions successfully on Fox Transfer decals - L
ONDON TRANSPORT and the yellow lining (7mm O gauge). Here I only applied gloss varnish to the area where the decals were to be applied. They were sealed with satin varnish and weathered.
These are Microscale and Champ decals I've applied to a 1:48 (O scale) boxcar with
HYDRA-CUSHION FOR FRAGILE FREIGHT being one decal. Here they are applied on a gloss surface.
Then varnished and weathered.