I completely agree that you shouldn’t depend on IP but if you’re experiencing delays dual path would help. Even if the cellular connection is great dual path makes for a better day to day experience as it’s lower latency. And dual path is more reliable than cellular alone as it increases the likelihood a signal will get through in time. But fair enough, it’s a personal preference and doesn’t address your real problem.
I agree that those are the 3 most likely causes of the problem.
There isn’t a good way to determine whether the tower is causing the problem. When we check known cell provider issues and we find a known issue in your area that’s a good indicator of the problem, but when we don’t find a known issue we can’t conclude that the tower isn’t causing the issue. The cell providers don’t always know. It’s a frustrating and difficult problem.
You can certainly try replacing the module or the panel itself. Between those I would replace the module first as it’s more likely to be the culprit than the panel and it’s lower cost but given that your signaling is good outside I think that’s a long shot.
Since signaling works fine outside your house then it seems like the signal is getting degraded or interfered with coming into your house to the location of the panel. I think the best bet, based on what we know, is to find a location in the house where the antenna works well, preferably within 10 feet of the location where you want the panel to be so you can just run the antenna there. The attic antenna does not need to go in an attic, it can go in the wall or even just on a window to help make sure nothing in the wall is interfering. Usually higher is better. If you’re comfortable drilling and sealing the antenna could even be routed through the wall and mounted outside in a IP66 plastic enclosure but that may be a bit extreme and there is the risk of a leak if not done well.
What signal strength did you see on the panel when you had it outside?