Adding my subdomain - followed guidance but no joy

Hi all
Hopefully someone can help - very rookie at the web hosting site of things.

I’ve set up fasthosts to host a very basic homepage and some redirects on domain thamesvalley.club. What I actually want is for glide to run on the domain but thamesvalley.club/sites/XXX (our old url) to redirect to a new glide site. This doesnt seem to be possible in glide?

To get round this Im setting glide to be on my subdomain app.thamesvalley.club with the fasthost server doing redirects, but glide doesnt seem to work on the subdomain, i just get the help screen red - any advice on what im doing wrong?

Fasthost subdomain

Fasthost subdomain DNS

Fasthosts main domain