A good start is to notice which planets were known to humans before telescopes. I hope this works. It should show you highlighted text showing that the Sun, Moon, and Mercury through Saturn were known before telescopes.
The visibility of a planet depends on how far away it is, how big it is, and what the outer surface or top of the atmosphere is made of. Mercury is close to the Sun, so at the right times it is quite visible. Venus has a lot of white clouds, so is easily visible. Saturn has its rings so is easily visible even though it is much farther away. Here are the albedos for the planets. Note that the moon is actually not all that reflective, with an albedo of 0.12. If it was not so close it would be hard to see. Neptune is farther away and less reflective than Saturn, so requires a telescope to see, even though it is one of the larger planets.
You could vary that a little bit by having your protagonist have either better than or worse than 20/20 vision. There was a famous vision test from ancient times. The second star from the end of the handle of the Big Dipper is a double star. Ancient people used ability to see this star as a test of good vision. So, you could have it so only people with very good vision are able to see your farthest planet. And maybe they get some special status as “sharp eyed.” Maybe that planet gets a name meaning “very good vision.” That could be a very nice little “business” in a story. Maybe other planets are named for other characteristics. Along the lines of Mercury for change, Venus for love, Mars for war, Jupiter for being jovial.
It must be the right point in the orbit of the home planet of your protagonist, and the right spot for the other planet. If the two planets are on opposite sides of the Sun, then they won’t be able to see anything. And if the orbits are very eccentric then they won’t be able to see the planet when it is on the outer leg of its orbit (the technical word for that point is the apastron). With Saturn having a 29 year orbit, if it had an apastron somewhat farther it might only be visible for, say 10 years, then invisible for 20 or so. And, of course, you would need to consider the moon. If your world includes a moon. When the moon is out it is tough to see even the brightest stars through the light it reflects.
Planets closer to the Sun are only visible at dusk or dawn. If you think about the geometry, they cannot be far from the sun or and are not visible in bright daylight. So, you get Venus as “the morning star” and the “evening star.” Some ancient people thought these were different planets.
It is odd you specify "at sea level." Visibility will be better at altitude. That might be another nice little "business" in a story. The mountain people and the valley people might have sharply different opinions about whether a given planet exists. There could be a lot of drama there with the valley people believing the mountain people were mad or something.
At this link you can find a table with the orbit radius and length of year for each planet. You can see some fairly regular patterns. And you can see why there is a "missing" planet between Mars and Jupiter. If you just "winged it" with this table, you could do OK. So if you want something with an orbit about 12 years, put it at Jupiter's orbit. If you want 20 years, you put it about half way between Jupter and Saturn. And so on.
Last thing. If you want to play with eccentric orbits, you can use the "equal area" law
to get the time it spends on each part of the orbit. You join the planet to the sun with an imaginary line and think about the area this line sweeps out during the orbit. It makes equal area wedges during each month. (Or whatever length of time you break up its year.) When the planet is farther away from the sun the wedge is long and thin. When it’s close to the sun, it moves faster, making the wedge short and wide.