Forgot to add, vikunja does have a mobile app but it lacks functionality. That said, the mobile in browser version of the app is excellent

Ok so I have a few options for you. Vikunja is a great self hosted option. I like it’s UI and the UX is very intuitive. However, you will need a domain name or rig up a VPN to sync it up properly. Another option is Super Productivity. Super Productivity enables you to sync between systems using something like Dropbox, proton drive, or even self hosted options like Nextcloud.