Tried this 4K HDMI Splitter 1x2 HDR D-o-l-b-y Vision Atmos i hope resolve your problem.
i used fire tv 4k max & NVidia Shield (old version) & vizio m512a-h6

1 Input to 2 Output

https://www.amazon.com/Splitter-l-b-y-Vision-Atmos-Scaler/dp/B07VP37KMB/ref=d_pd_di_sccai_cn_sccl_3_34/141-3584973-0795917?pd_rd_w=3EwmO&content-id=amzn1.sym.e13de93e-5518-4644-8e6b-4ee5f2e0b062&pf_rd_p=e13de93e-5518-4644-8e6b-4ee5f2e0b062&pf_rd_r=C07PQEWECPFWFJTZSE2D&pd_rd_wg=LtZl7&pd_rd_r=34c093b1-56c8-4994-8f67-7141ce216023&pd_rd_i=B07VP37KMB&psc=1

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2 Input to 2 Output

https://www.amazon.com/dp/B0BNSY3VKW/ref=sspa_dk_detail_2?psc=1&pf_rd_p=f734d1a2-0bf9-4a26-ad34-2e1b969a5a75&pf_rd_r=AWXXKAFJKH02SDC0QBP6&pd_rd_wg=egXbn&pd_rd_w=A3Z2L&content-id=amzn1.sym.f734d1a2-0bf9-4a26-ad34-2e1b969a5a75&pd_rd_r=6922de6d-2109-401a-9b03-7a87baab6cdd&s=electronics&sp_csd=d2lkZ2V0TmFtZT1zcF9kZXRhaWw&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEzQVZFQktXNTBPOFowJmVuY3J5cHRlZElkPUEwMjUxNzMzMTc4WUUzOUpCQ0hMMiZlbmNyeXB0ZWRBZElkPUEwMjk4Mjk1MTNNS0ZHRTBGWTVFNCZ3aWRnZXROYW1lPXNwX2RldGFpbCZhY3Rpb249Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU=