Mouse5 is neither working with my mouse so I had to use my mouse's drivers to map mouse5 to a key combination like shift+# (not sure if the game supports this automatically meanwhile but you maybe need to use an addon which enables the shift/ctrl/alt modificators to be used for eso keybinds ingame. Not sure which one enables this though) or any single key like +. Setting this key in the keybinds then will enable you to use the mouse5 or any other mouse key you have prepared this way.
Recommanded addons for keybindings:
https://www.esoui.com/downloads/info...nKeybinds.html
https://www.esoui.com/downloads/info...Keybinder.html