To enable neighbor discovery between the company router and an ISP that uses a third-party router, what is the next configuration step on the company device?

The correct next step is to enable LLDP globally. Link Layer Discovery Protocol (LLDP) is a vendor-neutral neighbor discovery protocol, making it suitable for interoperability between Cisco devices (like the company router) and third-party devices (like the ISP router). Cisco Discovery Protocol (CDP) is proprietary to Cisco, so disabling it on the interface (gi0/0) would prevent neighbor discovery if the ISP router was also Cisco, but it wouldn't help with a third-party device. Enabling LLDP TLVs on the ISP router is an action on the ISP's device, not the company's, and LLDP must be enabled globally before specific TLVs can be configured. Disabling auto-negotiation is unrelated to neighbor discovery; it controls how speed and duplex settings are determined.