Paypal subscription Pro Forms option not working on moblie

I have set up a membership subscription using s2member Pro Forms. The subscription is paid monthly and recurring. Registering a subscription on desktop goes to Paypal as expected but using the registration link on mobile results in the following Paypal error:

“We’re sorry, things don’t appear to be working right now. Please try again later.”

I have another s2member Pro Forms setup on the site which is Lifetime and Buy Now options in the shortcode, and this works fine on mobile.

I have changed the rt="" and rr="" to “L” and “BN” on the new form and that works on mobile too, but it won’t allow me to use rt=“M” and rr=“1” for mobile access.

Does that mean I cannot setup a monthly subscription for users on mobile?

Since I couldn’t get this working we’ve switched to Stripe Pro Payments which is working.