It is a fiber network and fiber has insane range thus it is unlikely to be attenuation. Bad device and bad cable are out because they are explicitly tested as described. Therefore TX/RX is the only reasonable option.

Explanation
A screened subnet is a network segment that is isolated from both the internal LAN and the Internet by firewalls. A screened subnet can be used to host internet-facing hosts such as web servers, email servers, or DNS servers. A screened subnet provides an additional layer of security and prevents direct access to the internal network from the Internet.
References: Network+ Study Guide Objective 3.1: Explain the purposes and use cases for advanced networking devices.

STP stands for Spanning Tree Protocol, which is a network protocol that prevents loops in a switched network by blocking redundant links. STP allows only one active path between any two network devices, and dynamically reconfigures the network topology in case of link failures or changes. STP is essential for proper stability of the network switches, especially when they are cabled in a redundant or complex topology that involves multiple switches and firewalls. Without STP, loops can cause broadcast storms, MAC address table instability, and duplicate frames, which can degrade the network performance and availability. References:
[CompTIA Network+ Certification Exam Objectives], Spanning Tree Protocol (STP) Explained | NetworkLessons.com

A major difference between a router and a Layer 3 switch is that Layer 3 switches are generally more efficient than routers for performing routing tasks within a LAN. They are optimized for high- speed packet forwarding and are typically used in environments where high performance is required. Routers, on the other hand, are designed for more complex routing tasks and often operate at lower speeds compared to Layer 3 switches.