Am unsure what version of Android/iOS my friend is on for his firmware himself, but I am on Windows 10 and for the last 12 months or so since Minecraft started doing pretty big updates like the bees etc. I haven't been able to stably stay in multiplayer sessions with friends on Minecraft. We will make worlds, play for an hour and if we get disconnected, or have to leave for an hour to go do something like groceries or pick a family member up, once returning and trying to reconnect to the multiplayer game it continuously flicks errors "Unable to connect to world." and many others? Doesn't exactly elaborate on what the issue is, whether its an internet connection issue or whether its something else? We have the same update of Minecraft, 1.17.10 and my friend has another mate who has joined and is on PC as well who had no issues joining or connecting to his world.
I have been experiencing errors and issues joining worlds and multiplayer realms since 12+ months ago and still no fixes are made and still I can barely play with friends on realms or their multi worlds because of these issues. I have an Xbox and a PC and it tends to happen on both, and my friends are generally on consoles whilst I've been on PC as of late so it doesn't make any sense considering its cross-play between Xbox and PC that I know of... I never used to have issues 24/7 with joining and playing with friends and since I have I barely even touch this game or have any incentive anymore. I have purhcased it separately for a total of like 4-5 copies of Minecraft and still feel since Microsoft has owned you - you've only gone further downhill.