Instance Timelines can be viewed independent from Hashtags. Use
view only public posts by users from that Instance.
The Hashtag Search needs an instance to work. Use a well connected instance like mastodon.social and choose Federated to get Toots with that Hashtag from other instances as well.
Thanks to @KaKi87@mamot.fr you can use MastoVue from within the Mastodon Web app through a userscript.