The system really depends on your budget. I know where you are located (I am in the same area) and cell service is pretty bad in that area. Unfortunately all the major carriers need additional cell towers as the current towers are overloaded both by radio capacity and back haul capacity. Things work OK during off season but when people come to town for Summer there is very little connectivity/data available. Also, as technologies change, cell companies have had to reposition antennas and focus on coverage on-land and that has led to loss of coverage over water unfortunately.
That said, you have to assess the carriers and bands. 5g in many cases does not go far, less distance than 4g, so you would likely not be using it away from the dock. We have hit or miss connectivity on LIS and go to Block often. In the middle of LIS you might not have any connectivity and once you get away from MTK you might lose as well. Personally, I’d focus on a CAT-12 or CAT-18 or CAT-20 LTE/4G device at this time and hit the pause button on 5G. There is no real 5g in our area any ways at this time and the technology/product offerings will change with time (NSA versus SA).
What plan do you have from AT&T that is 5G? Are you sure its going to work in a Peplink device? AT&T has very few plans that are compatible. If you somehow got your hands on FirstNET what ever device you pick make sure it has B14 support.
There are multiple versions of 20g and 40g. Are you talking about the Mobility or Maritime versions? Off the stern rail you could go either way but I’d recommend Maritime version especially if your boat will be heeling over, rocking, moving a lot. As for 20 v 40 model, the more connections you can have the better, so why would you go 20 if the Router you pick has four cellular connections? The Maritime 40 is pretty big but I would go that route.
Another question is do you want to connect to WiFi when you are at the dock so you are not eating up cellular data? If so, you should look for a device that supports WiFi WAN. You will need an additional antenna if using the Maritime version or could look at the Mobility 42G but again given your use case I’d avoid the Mobility series. The new Transit series might be worth looking into.
Finally, use high quality LMR400 cable, connectors, or similar. You can look at the tech sheets to see the losses across bands. A HD dome might be something you want to consider. Unfortunately Peplink’s new Dome has been MIA the past year or two. I am also not sure how the antenna polarization stacks up against a Maritime 20/40…
PS. there are many options for weather data. I had Sirius XM Satellite weather on my boat in addition to cellular data. Viasat and other providers also offer data/weather options. As a fellow Boater, I would suggest you have multiple options/redundancies. I subscribed to Radarscope Tier 2 for weather and it is one of the best options out there for Radar/etc. A lot of meteorologists use it and it takes a little bit of learning.
PPS. I would even consider a dual modem Peplink and get SIMs from all the major carriers (VZW, ATT, TM). That will improve your connectivity big time especially in difficult coverage use case. You would possible need an additional antenna for the second modem but if you dont use all four for one modem, you can use the other two for the second modem.