Solo bass practice is necessary. Bass in a band is a different sport. You're not the lead instrument. Your job is to make the band sound better, often by playing less than you could.
Listen first, play second
The cardinal rule: listen to the rest of the band, then play. If the singer is mid-verse, simple roots. If the guitarist is taking a solo, simple roots. If everyone is laying back, that's when you can add a walking line or a fill.
Three modes
Foundation mode: roots, locked to kick drum, all quarter notes or simple eighths. This is the default for 80% of any song.
Walking mode: 1-3-5-octave or walking patterns connecting chord changes. Use during instrumental sections or transitional moments.
Fill mode: a short melodic phrase at the end of a measure to set up the next chord. Use sparingly, maybe once every 8-16 measures.
When in doubt, less
The best bass players play less, not more. Listen to Stand By Me: Tommy Cogbill plays the simplest possible bass line, just root notes in a 1-vi-IV-V pattern. The song is iconic. The bass is iconic. Nothing fancy at all.
The temptation when you're new is to fill every space. Resist. A bass that breathes lets the rest of the band breathe.
Eye contact + dynamics
In a real band: watch the drummer. Drummer hits the snare hard, you can hit harder. Drummer plays brushes on a ballad, you play soft. The bass and drums communicate without words.
If you're practicing alone, you can simulate this by playing along to records and matching the dynamics of the original bass part.
80 BPM. Pick a song you know. Play only the root note on beat 1 of each measure. Just one note per measure. Listen to how spacious that sounds. That's good bass.
Next: putting it all together.