As Jodokast96 posted modems tend to either work or not, if it fails it tends to fail totally.
First step is to access modem stats to see if modem is able to sync at correct rate. Since Verizon said line looks fine problem most likely is not physical DSL circuit but network congestion between customer and remote server.
Next step is to do line quality test on the BBR tools page and/or do Traceroute (tracert in Windows) to stable sites like BBR. Round trip time should gradually increase with hop count and distance. If you see sudden increase it may indicate congestion at that or previous hop.
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