So you are testing via a wired connection direct to the router? Have used multiple nearby speedtest servers and done a basic traceroute?
Spitting your dummy out only benefits OR/PN. Unfortunately refusing a further OR visit is going to prevent them from investigating/resolving the issue and leaves you uncompensatable as you’re actively refusing reasonable attempts to resolve the issue, it would also be ironic if switching ISP’s yielded no change and still required a OR visit.
My current BT issue started on Black Friday (November) and has resulted in at least 7 appointments of which BT have failed to attend or booked out the wrong engineers to all but two. I’ve told them how to resolve it since the 21/12, even had a final deadlock letter from BT stating i’m wrong, only for them to suddenly cave in and agree with me a week ago after going an indirect route via a 3rd party/the press office, this is at CEO’s office level, you’d hope for better, but then again i’m paying a pittance for what a I get, if I was paying for a business class service with SLA, things would likely be different.
Your options are to accept OR will need to attend to properly investigate the issue, move ISP and pray to whatever imaginary friend you believe in that it’ll work as it did pre-fault, accept the fault and live with it, or find an alternative method of connection, the choice is yours