T-Mobile, AT&T, and Verizon all provide strong 4G and 5G throughout the San Diego metro area. The Gaslamp Quarter, Little Italy, Hillcrest, North Park, and the downtown waterfront all have excellent signal. Balboa Park's outdoor museums and gardens have solid 4G.
Mission Beach, Pacific Beach, and Ocean Beach have strong coastal coverage. La Jolla Cove, La Jolla Village, and the UCSD campus area are well-covered. Coronado Island and the Silver Strand connecting it to the mainland have reliable 4G.
The San Diego MTS Trolley system has surface coverage along all three lines. Note that the US-Mexico border crossing at San Ysidro brings your phone into Mexico signal territory if you venture close, so keep your home SIM's international settings off.
Torrey Pines State Reserve trails have signal at the trailhead parking area but can weaken on clifftop trails.