My pages.dev site got certficate issued with root certificate gts root r4 which is not in many devices it can be tested by using old mobile device or using webpagetest.org default mobile device edit- not workinf in pcs also check webpagetest.org

This is a known issue with the GTS CA and was fixed after your certificate was issued. You can also make a post on https://community.cloudflare.com and someone will be able to change it for you. You can also wait for it to renew by itself