Warranty replacement for SB8200

Modem purchased May 17, 2022.  Worked until recently when internet connection would drop several times a day.  Resetting modem several times would restore connection.  ISP checked their equipment and replaced incoming cable because speed was a little slow.  This did not solve problem.  Due to visitors arriving in a few days I purchased a new SB8200 and this resolved problem.  Original modem was purchased at Walmart for $169.00 but they were out of stock when the unit began to fail and replacement was purchased at Best Buy for $138.99.  Both retailers told me to contact the manufacturer to recover the replacement purchase cost.  How do I proceed with this warranty claim?