Internet extension to outside building

A "wireless access point (AP)" will do the job.

Another router could lead you into possible problems, with two boxes vying to control the network. You would have to configure it as an AP, which could be more expensive and would lose many of the features of genuine router.

You might do better to buy a wireless range extender that offers Access Point Mode. The EX8000 is probably the top of the range with the WN2500RP at the cheaper end.

Start here:

WiFi Range Extenders: Boost Your WiFi Range | NETGEAR

Unless your office is massive, the wifi range doesn't have to be that good, enough for one room, unless you plan to sit out in the back garden soaking up the sun while you work.

I'd look for things that I could wire to the AP, especially the smart TV, Mac and printer. Easier to set up and manage than wifi connections. Keep that for the smartphone.

I forgot, you could also investigate the Orbi range. Aimed more at the consumer, and supposed to be easier to set up, it is a bit less capable on the wired device front. But a cheap ethernet switch could deal with that if you are keen to wire stuff up.