Says "Unable to Connect To World" When I Try To Join a World

I was trying to join my little brother's modded Minecraft world. We both play on mobile and we were sitting next to each-other. I tried joining and after it said 'Locating Server' it showed a screen saying 'Unable to connect to world'. We tried joining a non-modded world but the same thing happened. This issue happens every so-often, like, one day he can join me the next day he can't. We're also always on the same version, which doesn't make sense. This has been happening to us for a while now and I just recently got a Jaro (or whatever the account system on this website is called) account so I could report this. Anyways Mojang, please fix this.