Transmissive reconfigurable intelligent surfaces (TRIS) have emerged recently as promising relay technologies for 6G communication, mitigating the signal attenuation caused by outdoor-to-indoor (O2I) propagation loss. Typically installed at the O2I boundary, TRIS can enhance the connectivity between outdoor base stations and indoor devices. This study experimentally investigates a TRIS-based relay system for O2I communication using IEEE 802.11ax (Wi-Fi 6) signals at 5.5 GHz. Experimental results demonstrate an improvement of up to 19.5 dB in the received signal power compared to scenarios without the TRIS. This improvement surpasses path loss model predictions by 11.8 dB due to multipath effects. Additionally, the measured error vector magnitude (EVM) met Wi-Fi 6 standards (below −30 dB at MCS 8). These findings validate TRIS as effective solutions for addressing shadowed areas and improving the signal quality in O2I environments at 5.5 GHz.