Are you in the US or the UK? The device selection is quite different.
And are your Hue bulbs connected to Hue bridge? Or directly to SmartThings?
If you are using a Hue bridge, that forms its own mini network and it doesn’t matter what protocol the bulbs are on–they will only repeat for each other and won’t interact with any of your other devices.
Since you said you were using iris door sensors, I’m going to assume that you’re in the US. In that case, a good zigbee repeater is the iris smart plug that works with SmartThings. That will be the second generation, model 3210-L. This is very popular in the community, and is a good reliable zigbee repeater. It’s also a very unusual device in that it has two radios, and also operates as a Z wave repeater. This is one of the reasons for its popularity.
https://www.lowes.com/pd/Iris-Iris-120-Volt-White-Smart-Plug/999925330
The securifi peanut plug is typically sold under $25, and sometimes even right around 20. It should work fine.
https://www.amazon.com/dp/B00TC9NC82/
GE makes both a Z wave and a zigbee version of their pocket socket. Z wave model repeat for Z wave and zigbee model will repeat for zigbee. They look identical, so read the box is carefully to make sure you’re getting the one you want. These were usually sell for between 30 and $40, although sometimes they will be on sale for less. This is just a good standard basic pocket socket.
https://www.amazon.com/dp/B01E9GKOWO/
And of course as of this writing many places have both versions of the SmartThings plug on sale for Black Friday. Either should work fine as a zigbee repeater and with the sales these may be down in the same price range as the iris or even the peanut.
Architectural Issues
All of that said, the problem may be that exterior wall. Exterior walls often have architectural materials which significantly block signal. Sometimes you have to get really tricky to get signal past them. It’s not just a matter of distance.
There are a couple of Threads in the forum on dealing with these kinds of issues, have you had a chance to look at those? It may be more a matter of placement than the specific device .
The following thread is sort of a round up topic and has links to several of the other threads that discuss different approaches. It’s a common challenge for standalone garages. Although the question being asked is about the wave, the same types of placement solutions will apply for Zigbee. There’s usually a way, but you may have to try several different approaches to get one that works for you.