We also have an issue playing HBO Max through Roku to our TV. It is usually the movies that don't play, the TV series usually play with no problem.
We have a Roku Ultra 4670x (only a year old) and it's hard-wired into a switch that's hardwired into a 1G cable modem. It's also updated to the 10.x OS, so I know that's not the issue.
There should be no issue playing the movies. We also have a similar problem with Paramount+.
I've done the 'remove, restart, reinstall, restart but that hasn't helped.
We have a 4k LG Smart TV that's only about 2 years old.
Wife is getting very upset that she can't watch movies 😞
I'm going to try to contact HBO and see what they say.
You didnt mention your firmware version+build (Settings/System/About). There are many builds of OS10, so you need to be more specific.
Can you also be more specific when you say "movies dont play" for each app?
Since you mentioned two different app/services that you are having issues playing movies with, its likely to be a configuration issue with your Roku or TV (LG model unknown).
Did you configure your LG's HDMI ports for "HDMI Ultra HD Deep Color"? Settings/General/HDMI ULTRA Deep Color=enabled (for each port).
You might also manually configure your Roku's display: Settings/Display/4K TV or 4K HDR 60fps TV