Your voice is clear in a call, but a held guitar note fades away. A faster connection is not the first thing to buy. Speech-focused processing may be treating part of the instrument as unwanted background sound. That is a possibility to test, not a diagnosis from the symptom alone.
Compare one short passage
Ask someone in a test call to listen to a spoken sentence followed by a sustained note. Keep the microphone, its distance and your playing level unchanged. Compare the sound before and after changing one audio setting; your own headphones do not tell you exactly what the other person receives.
In Microsoft Teams, the current music-mode instructions start at the arrow beside Mic in the meeting controls, then More audio settings → High fidelity music mode. Available controls can depend on the app and your organisation’s settings. Music mode is not available with spatial audio, according to Microsoft.
The mode is intended for music. Microsoft recommends good external audio equipment and advises against Bluetooth devices for this use. It is not a promise that any microphone will capture an instrument well.
Keep the test useful
Do not switch off every audio effect at once. If you try disabling echo cancellation, use headphones and listen for echo before continuing. Record which setting improved the passage so you can reproduce it for the lesson.
If music mode does not help, check the selected microphone and possible input overload next. Return to ordinary call settings for conversation; music mode can use more bandwidth and let more room noise through.